There are three basic components to preparing for cross-country mountain bike racing. Athletic performance, technical skill and race wisdom can be emphasized as a combined program or isolated for working on areas of weakness. It doesn't matter if you are a casual weekend racer or developing a more serious racing career; these training elements are essential.
Besides needing cardiovascular superiority, you need endurance and explosiveness to be a competitive force as a mountain biker. Road cycling or utilizing a stationary bike is an excellent way to increase cardiovascular output as well as endurance, especially when you do intervals as a part of the riding. The reason this is such an effective way to train is that the pace is continual, whereas mountain biking frequently requires pausing to navigate trail obstacles. Typical training rides should last between 45 and 120 minutes, and interval training for endurance should last between 15 and 45 minutes, depending on your goals. You can also improve endurance by cross-training in sports such as such trail running or cross-country skiing.
Weight training using leg extensions, leg curls and squats is an excellent way to tie it all together by increasing endurance and explosiveness. For extensions and curls, use low weights of 20 to 40 pounds with four to six sets of 20 to 50 repetitions. For squats, use weight at or around your body weight and do a high number of reps. Performing jump squats is also an effective way to improve explosiveness and, if you do them regularly, they can replace or decrease the need for weight training. Besides adding power to your muscles, these exercises help you gain a higher lactic acid threshold and increased pain tolerance.
No matter what way you train, always push yourself a little harder and a little faster each week. The most common error in training is becoming complacent with the routine.
Technical skills are the bread and butter for mountain bike racers. Negotiating rocks, roots, drops and mud in an efficient manner presents a major problem for most beginners. This is the most difficult area for training, but can pay big dividends for the time invested. Technical riding combines uncanny balance with split-second decision-making for negotiating obstacles. Unlike walking a tightrope, which requires great lateral balance, mountain biking also requires superb forward/aft balance. A balance board can be used as the first stage of developing better balance. For the second stage, practice standing on a balance ball. This is a difficult task, but may be made easier by holding on to a rope for balance. With practice you'll eventually discard the rope. A slack-line is similar to a tightrope, but is more stretchy and dynamic by nature, which helps you practice forward/aft balance as well as lateral. Many indoor climbing gyms have slack-lines available for use.
To conquer the most difficult obstacles, there is nothing like practicing with the bike on real trails with real technical obstacles. Don't be afraid to repeatedly ride a particularly difficult section until you get a feel for how to master it. This happens through trial and error, but teaming up with a friend who has mastered these skills can dramatically improve how quickly you learn. The other method for mastering technical skills is the exact opposite: Instead of learning how to roll over the obstacles, pick a line that presents the easiest path from point A to point B and avoids difficult obstacles. This requires learning quick serpentine steering and making quick decisions. This is also best practiced on the trail or by setting up a tight slalom in a large yard or field.
Entering a few races for the purpose of training is essential for truly being a prepared racer. Developing race wisdom is most likely the most neglected area for new racers, but it frequently makes the difference between placing in the top 20 percent instead of the bottom 20 percent. Some of the things to learn are how to prep yourself and your bike for the race, how to warm up and how to avoid flats and damaging your derailleurs. This is best learned by teaming up with a more experienced racer, because the specific tips and tricks change with technology and the times.
The best way to gain race wisdom is to locate established racers either at the local bike shop or in an online forum. See if a racer is willing to ride a few beginner-class races with you to help you learn faster. This process will save frustration from learning the hard way.