CLIMB FASTER BY DESCENDING BETTER
Especially for newer cross-country mountain bike racers, it is important to spend time riding off-road to build skills and expose yourself to new terrain and situations that you may be faced with come race day. However, it's no secret that a more on-road-oriented training program will yield more consistent, quality workouts and, because of this, is better for building fitness. So how do you balance between the two? Make your time spent riding off-road count as much as possible.
In the off-season, spend one ride each week working on specific skills, typically on a day that they will also spend some time in the gym. An hour or two working on bunny-hopping, cornering, drop-offs, jumping, cyclocross-style dismounts/re-mounts, or other skills will generate improvement faster than working on the same skill in a less-structured way. Basically, you maximize improvement in a given skill versus the time spent riding off-road. This way, your skills and confidence improve faster, and you have more time to dedicate to on-road training to make maximum fitness gains.
Obviously, improved skills and confidence will make you a faster rider in technical situations, but it can also improve your climbing... The better you get at descending, the more relaxed you will be while descending during a race. You will save valuable energy that can be applied to the climbs later on.
Also, as your DH skills improve, you can cheat your bike setup to a more climbing-friendly configuration, without giving up time to your competition. Maybe this means you use a more stretched-out position on the bike, or get rid of that wide handlebar your bike came with, or use less suspension travel, all of which can help you climb better.