In my hometown Canberra, some of the best XC riders are downhillers, especially when it comes to short-track racing. I put this down to not only their aggression, but the skills they gain from "playing" on their bikes. In Canberra you will always find downhillers hitting up the jump park, carving up the urban terrain and basically just fooling around on their bikes. This is not only fun but it improves their skills by improving their balance, improving their bicycle awareness and overall, increasing their ability to handle their bike. How do I know this? I used to be a downhiller/free rider and I used to do all of these things. I used to spend hours on the road out the front of my house working on flatland tricks and just playing. When I made the transition from DH to XC, the skills I had were extremely useful and were applied immediately to make me fast through the single track.
In the higher divisions of XC racing in Canberra, there are three types of riders: those with fantastic fitness and above average skills, those with above average fitness and fantastic skills and those who have fantastic fitness and fantastic skills. The first two riders described are usually pretty even but guess who wins the race? the rider with the lot. I believe that in the sport of mountain biking, skills are just as important as fitness.
Improve your skills now
Now I'm not saying you need to go out and buy a DH bike or Jump rig. It's probably better that you play on your own bike anyway so you can get to know it more intimately. Do what I do and get on a flat surface and start practicing tricks. Wheelies, nose stands/endoes, track stands and bunny hops are a few to get you started. when one skill becomes too easy, take off one hand/foot for a greater challenge. Get creative! When you start to get creative on flat terrain, you will use your creativity more on the trail when it comes to line selections or getting yourself out of trouble.
Find (or build) a little table-top jump to practice getting air. I have found that confidence in the air breeds confidence when on the ground. Confidence is one of the key ingredients when it comes to going faster. Having the ability to get air with confidence is great for clearing obstacles, jumping over rock gardens and taking faster lines.
Changing your bike setup for playing isn't crucial because after all, it's just playing. The only things I may suggest is lowering your seat to give your rear end more clearance and switching to flat pedals (I don't usually bother but if you are new to clip less pedals or have poor balance, it may be a good idea).
When to play
I'd aim to play on your bike at least once a week of you can. you can incorporate it into your training as active recovery or even dedicate it as a skills session. It's always fun if you do it with some friends and riding with other riders will always push your skills!
BMX track riding for supreme single track skills
ride slow to ride fast