The Bike
You will need a bike that produces a good amount torque at fairly low revs (between 3000 and 6000 rpm), a gearbox that has a longer first gear. You’re also looking at and a maximum speed of less than 120mph. The suspension needs to be stiff and high with reasonable off road tyres. The handlebars are typically wide and upright. Tyres are inflated a bit softer than on the road. A good all round pressure is 1.1 bar (15psi) for the front with the rear being a bit harder (increase tyre pressure above 2 bar however when traveling over very rocky terrain in order to protect your rims and avoid punctures).

If you plan to ride more challenging trails your motorbike should have wheel rims with spokes and heavy duty tubes which are able to resist objects like thorns. Inner tubes should be replaced every 12,000 miles or so, even if they have never sustained a puncture. The reason for is the area around the valve deteriorates and can tear causing a puncture that can be impossible to repair. Water that seeps in through the spokes also causes damage to the rims, spokes and tube. Ideally your rims should dry out before you park your bike.

The vulnerable parts of your bike should be protected by engine protectors, crash bars, bump plates etc. front and back. Other equipment e.g. headlights, brake calipers, turn indicators, radiator etc. should also be protected.

The front suspension stanchions need to be kept clean and damage free otherwise the seals become damaged. This can result in the oil in the front fork leaking. If you intend to do a lot of off road riding you should consider protecting these areas with rubber boots and replacing the standard weight suspension oil inside them with the heavier grade.

Boots that offer solid ankle support (trainers and sandal are out)
Long trousers that are tear resistant.
Knee protection. These can be built into the construction of the trousers or worn separately and externally over the knees.
Thigh protection. Usually built into the trousers
Jacket that offers elbow and shoulder protection

Some basic off road riding skills
These can be split into the following:

Look up - look ahead. Look beyond the stuff in front of your tyre.
Keep the throttle open!
Lean forward - arms relaxed and bent
Keep the bike perpendicular (a right angle) to the road surface
Use the back brake and stopping power of the gearbox.

2. The need for speed
This is difficult as the natural reaction is to travel too slowly. Your path may be covered in loose stones, grass and sand. Avoid the temptation to close the throttle. Don't do it!! Slow down a little and then just before the loose stuff starts accelerate gently. If the bike twitches and moves around add a bit more throttle Once you are through the bad patch you can slow down a little if you want to.
The amount of stability a bike has depends on its forward motion. More speed helps it punch through and over the bad surface. Generally as the bike travels faster and faster you will find that it becomes more balanced, stable and maneuverable. Going too slowly over rough terrain will cause the bike to move around more and perhaps cause you to panic. Therefore, within reason, go a bit faster than you are comfortable with.

Because you need to keep the throttle open when traveling through the loose stuff do not approach bad patches too fast or in a gear that leaves you in the middle of the power band. Choose a lower gear at the bottom end of the power band so that you can accelerate gently!

Grip the tank firmly with your knees and push down onto the foot pegs with the balls of your feet. This makes you and the bike into a single unified unit where your body weight is as much part of the steering process as the front wheel. Your upper body should be relaxed and free to move about easily.
Should the surface of the road deteriorate even more, keep the gas open!!! A decelerating bike will flounder. Push down hard on your foot pegs. You will naturally find yourself standing up which is the correct thing to do. Do not use the handlebars to pull yourself up - their job is to steer, not pull!!

4. Keep the bike upright when cornering
Your first ride off road is not the place to counter steer and lean your motorcycle into the corners! Instead keep the motorcycle upright. Yes, this will mean going slowly around the corners but speed can come later.

As you gain confidence you will want to corner more quickly. The secret here is to get lots of downward pressure on the outside foot peg as you corner. This keeps the motorbike firmly on the road and prevents the centrifugal force of cornering from throwing the back wheel outwards. The easiest way to do this is to stand with your weight on your outside foot. In this position you can even lean the bike into the corner

Very fast cornering off road using the accelerator to twist the bike around is an advanced skill and can come later and is not covered here.

Approach corners and difficult patches with the old adage of”Slow in and fast out".
Keep your eyes up. Look ahead.
Ride defensively, anticipate problems.

Your gearbox and rear brake should give you all the stopping power you need. Unlike riding on road, locking your rear wheel up on a lose surface is not a problem. If you need the extra stopping power you can progressively pile on the front brake until it shows signs of locking up - then ease off a little

Look out for blind corners and brows of hills
Once you have mastered the basics of off road riding your next challenge is to recognise the dangers of blind corners and brows of hills. The rule here is to maintain a speed that is in relation to the amount of visible track ahead of you. What you cannot see, you cannot avoid - if you are going too fast when you do see it, you will be in trouble.

This is as relevant on road as it is off road.

Going up short and very steep hills
Being potentially hazardous, short very steep up hills deserve special mention. Before you commit yourself to going up you have to be sure that a combination of your skill, bike, power, tyres and track surface leave the odds in your favour! Getting half way up with the bike stalling and falling down on top of you is not a place you want to be! You also need to know something about the terrain at the top. If you power up over the lip you will have little chance of stopping if there is a 20 meter drop on the other side.
Getting up a steep incline successfully depends on the preparation you make before you actually start.
The technique is broken into two separate phases and applies equally well to mud as to uphill sections

1) Preparation phase.
Is this a wise decision? If 'yes', you must be totally committed!! Choose a line up the incline. Choose a suitable gear. Increase the revs and move into the power band (torque). The bike will accelerate but moderate it to a suitable speed. Move your weight forward over the front wheel. For a quick short incline you can remain seated but for a longer one you may need to stand up on the footpegs and move your weight forward. (Do not rest your weight down onto the handlebars as this prevents you from steering effectively.)

2) Keep it going phase.
Keep the revs a little below the power band. Ease off the throttle if you have too. If the bike’s revs begin to drop, ease the throttle open. You do not want the back wheel to spin and loose traction. If the revs continue to drop change to a lower gear. (This could mean that you approached the incline in the wrong gear in the first place!)

If you find you have miscalculated and the terrain in front of you becomes too steep, aim your front tyre for a ledge, boulder or pothole and 'hook' your front type over / into it and anchor it there with the front brake. Lay the motorbike down on its side, move it down to a safer area before attempting to ride it back down again.

1) Preparation phase.
Stop (or pause) at the top of the hill and choose your line. Drop down into first gear and let the clutch out. The bike will speed up and move over the lip of the descent! (Scary Stuff!). Stand up on the footpegs, just enough to get your bum off the seat but keep your weight as far back as possible. Place one finger on the front brake lever.

2) Keeping it going.
The motorbike will 'plunge' down the first few meters but suddenly the bike's speed will level out as the compression increases. Navigate your way down, arms bent and relaxed, gripping the bike with your knees when the needs arises. If the bike does move too quickly (or you need to slow down for a tricky maneuver), gently squeeze the front brake lever with your one finger. (It has a lot of downward pressure on it and is unlikely to lock up.) If the surface is very loose, dab the front brake on the good stuff and release on the bad. Despite moving slowly and not giving it any throttle, the bike will not stall unless you stop.
The bike will move briskly down the hill in a very controlled manner but now you find you are heading for a turn!!! Release the brake at the last moment, push down hard on the outside footpeg (you are still standing?) and with a positive movement, dive the bike into the turn. Hey Presto! Despite your worst fears, the bike will turn the corner.

The whole manoevre just takes commitment, confidence and belief that the bike can deliver. Remember, that a moderate amount of speed is your friend. Your bike becomes light and maneuverable without stressing the limited traction too much.
NOTE: The back brake, although a more logical choice has very little downward pressure on it unless you have a pillion passenger. It therefore locks up too easily and offers very little stopping power.

Tips
Allow the motorbike to follow the channels and ruts in the road and do not fight the bike out of them.
Keep the bike below the power band with a somewhat aggressive attitude. This way the bike will not falter and you will have enough power at hand but without the danger of spinning the rear wheel.
Riding over bad transverse corrugations on a dirt road is no fun as it rattles and shakes both bike and rider around. The are two tips: 1)go faster, rather than slower as this tends to 'smooth' the ride over the tops of the corrugations. 2) change to a lower gear well below the power band (maximum torque). The lower revs will prevent the engine constantly 'snatching' at the power and as a result, also help smooth out the ride.

Check out the horizontal position of your hand levers (brake and clutch) relative to your handlebars - they should be lower. Adjust them if necessary. This keeps your wrists higher than your fingers on the hand grips. Should you have a spill, the force of your body moving forward will not be transferred to your wrists. It also means that when you stand up when riding over a bad section you will still be able to easily operate the controls.

All gravel roads have strips of loose sand and stones usually down the middle of the road. Treat this as you would any bad section. (see above) Cut across it with a firm, decisive action. Approach it with a steeper, rather than a flatter angle and combine with a bit of throttle.

Keep the sole of your boot horizontal with the ground while on the footrests. You do not want the front of your boot to catch on a protruding object like a tree trunk or boulder.

Tracks covered in loose rocks bigger than tennis balls are difficult to traverse and should be left until you are more experienced although the riding skills needed are essentially the same.