The best place to start is making sure we understand what Cross Training is. Putting it simply, it's adding another exercise to the one that you are racing in so for example you are racing XC and add a Pilates class to help with flexibility and strength. Yep cross training is really that simple.
Now, I'm sure we are all aware of how little time we have to train so why would we want to spend time not on our bikes? I totally get this and yes when we train we need to be specific and so being on our bike is the most specific training we can do - though that is in an ideal world. We could add a 15 minute strength session 3 times a week and this will give us physical gains as long as we keep the exercises specific to MTB.
Check out this Nino Schurter link on how specific he has made his strength sessions.
We can use items around the home to help us to train and so saving time and money. We have mentioned before single leg step ups on a kitchen chair or hopping up the stairs and these actions mimic those of pedaling, making the muscles fire in the same way.
Sometimes I work with athletes that need a little push and so booking onto a local class of Pilates or dynamic yoga means they go and get these extra sessions done. If they were to try and do these at home, they would not happen. This comes down to the individual and how your motivation works.
I should mention at this point that some classes are not always the best thing to do, the CrossFit phase we are going through at the moment does not always promote good form and technique in exercises as movements can be rushed and can actually cause you more damage, so always try and think about specific ranges of movement and make sure they are controlled.
It will also not be practical to have over sized glutes and biceps that will hinder you on the bike.....
We must try and remember that as well as our focus on training for MTB/XC we need to be able to function in life situations and cross training can help protect us by adding strength to joints for simple things such as carrying shopping, moving furniture and so on, these are the things that will normally cause us an injury and thus mean time of the bike - the staying fit for life as well as sport is important. The flip side of this is also it means we can protect ourselves better in crashes by having more muscle. It acts like armour around us and although it still hurts, it protects bone, ligaments and vital organs.
Before we sum up, adding in exercise off the bike actually gives your mind a break and helps to keep you fresh and motivated to ride... Our minds need to be looked after as well as our bodies.
To sum up we need to look at our training holistically, it's not just about jumping on the bike and turning the pedals anymore.