I happen to be one of those who attract new riders into the sport, and have given the basic training to many people. Here is a brief of what I try to teach them:
- Riding position
- Be ready to react
- Have your bike properly tuned/fitted
- Scan the terrain(Look forward)
- Use a proper braking technique
- Never get to the extremes
- Grow progressively
- The most important: never stop trying/practicing!
The attack position
The position that works best for me is slightly raised from your seat, knees bent, lowered torso with elbows bent. Feet at the same height with the dominant foot on front.
Ready to react: Your hands should be firmly gripping your handlebar, but not tense. One or two fingers ready to activate brakes. Choose the fingers you want on the bar based on whether you get the best grip. It is a little more important to control the handlebars than to freak-ly squeezing the brakes all the time. This is critical over rock or roots sections that shake the bike in all directions. You need to hold those bars captain!
Besides this, your elbows and knees should be slightly bent at all times, giving you chance to extend or contract them as needed, so you absorb terrain irregularities as you pass them over. It is like moving the bike around you (up/down, front/rear, right/left). This is why I do not recommend to sit way back arms fully stretched: In this position you have no freedom steer or to lower the bike while entering a hole or a "step down". An easy way of thinking is this: Your arms and legs are "your" suspension, you want to be positioned rather in the middle of its travel.
You should tune your bike as best as possible. This includes brakes and suspension (if any). Your brakes should be at best performing shape. This is a whole complete subject, so I wont go deeper on this, but basically, they must have won your confidence. The levers should be placed where you can be more effective to apply the brakes. The distance between the brake lever clamp and the handlebar grip should allow you to actuate the lever with the index finger, the middle finger or both. The "reach" is how far is the lever from the grip when brakes are nor being applied. This distance should make comfortable for you to grab the lever, not too far that it is difficult, not to close your fingers get tangled with it. Also, when you apply the brakes, the lever should go down to the position where your fingers feel stronger doing so.
The suspension should be properly adjusted too. The two main adjustments are preload and rebound speed. Preload is how "stiff" is the suspension and it determines "sag" Sag is how deep the suspension travels on rider weight. A proper amount of sag allows the wheels to travel down when required. When you have too little sag your tire looses contact with the ground too easily and you loose control. Too much sag makes the suspension makes it easy to bottom down, hitting the far end of the suspension travel. This may also lead to loss of control or another failures.
The rebound speed is how fast the shock absorber returns after hitting an obstacle. too fast and the bike will feel shaky (you'll feel no confidence) and maybe it will bounce off the ground loosing traction. Too slow and it will feel like getting stuck after a bump, effectively it will not be ready for the next bump if it comes too quickly. The correct intermediate position depends on your riding style and the particular terrain characteristics.
There are other adjustments that can be made, but they vary a lot model to model, brand to brand, so the best approach is to refer to the user's manual.
Scan the terrain
Look forward, never look down to the tire. The faster you want to go, the further up the trail you must look. Scan the terrain and look for the best line. Choose areas where you have more traction, or surfaces were is better/safer to brake. Dry solid rock or compact ground are commonly good options. There is a saying that the bike will go wherever you are looking at, so, focus on your route, choose where you want to go and stay there! It is common for a rider to stare at that rock sticking out, having fear of it, and... crashing into it. Avoid that behavior, don't stare at obstacles, but trace your path around or over it, and focus in that.
The most important skill regarding brakes (IMHO) is knowing when NOT to apply them. Braking is instinctive, as soon as you feel danger, you'll squeeze the brakes. But there are a lot of situations where it's best not to. First of all, never squeeze the levers quick and hard: this makes you lock up your tires and loose traction. You have to be gentle. Usually you have to release your brakes when going over wet roots, slippery rocks, etc. Specially you should release your front brake just before hitting a rock or a hole, so it will be free to roll over the obstacle and thus avoid being tipped over the bars.
I recommend to use BOTH brakes at the same time. You'll find a lot of divergent opinions about this but, I've found that each brake serves a purpose. The rear brake alone can slow you down just a bit, but it tends to straighten the bike while over slippery surfaces or loose sand/gravel, a little like the rudder on a plane. The front brake can stop you down, but it tends to trow you to the front. It is vital to learn to brake without locking up the wheels, specially during a turn. If you are focused on where you are headed, it is more dangerous to over-brake than to brake a little less. It is also important not to hold the brakes all the way down, thus avoiding overheating, which can lead to brake failure.
Never get to the extremes
Never adopt extreme postures. Not to far back, not to low. I made such mistakes at my beginnings, but I had to correct those. Stretching to be too far back lessen your ability to maneuver. Using the seat too low for "improved stability" also puts your legs in an awkward position that does not let you negotiate ups and downs. While going downhill over the roughest section you should be off the saddle to avoid hitting your spine anyway.
Never try too hard to get up to your buddy's riding level, do not take risks that are beyond you skills. Go a little at a time. Practice a lot. Find the way of practicing the obstacles over and over. Grow in technique before you grow in speed. Each kind of obstacle requires specific movements, as you learn them, you need to build "muscular memory" before taking it to another level, so be patient, but never stop trying/practicing. Be humble and practice even what you feel you've mastered. When you conquer an obstacle/jump/drop etc. for the first time, try to go back and do it again, and again, and again...
You may have already built skills in some of these areas, but each one of these points has way more to discuss about. I'll be happy to go deeper in any subject if I find questions directed more specifically towards any of them. My credentials: I am a cross country and downhill rider with a few podium positions at local races (none is first place, yet) ;) .