Is the bike under any sort of covering? What kind of bike do you have? Full suspension or hardtail? Low quality or high quality?
Regardless you'll need to at least keep the chain cleaned and lubed if you want to get any kinda life out of the drive train. They sell fancy chain cleaners and lube at the bike shop but they are expensive. Then again chains, cassettes, and chainrings are much more expensive. A clean and lubed chain makes quite a difference on the trail believe it or not. Plus it doesn't wear out the rest of your components as quickly.
You'll also want to keep your cassette (rear gears) clean of excess gunk. Gunk build up affects your shifting quality. I spray my cassette with simple green, scrub it with special park tool cassette cleaning brush, hose it off lightly, avoiding direct spray into the hub bearing/axle area and dry it off with compressed air. I use a park tool chain scrubber/cleaner $$$ with bio citrus cleaner because you don't have to remove the chain from the bike and it's ok for the enviroment. That also saves like $3 a cleaning for the Shimano master pin that is required for Shimano chains. Sram chains are simple because they have a master link that technically doesn't require any tools to remove/install. Make sure to run the bike though all the gears and then wipe off the chain before riding it on the trail. Excess lube just attracts dirt.
Less often but not to be ignored are your brake and derailleur cables. Minimum, you should get a good lube from the bike store and apply it to the openings/ends of your cable housings while moving or actuating the cable. You can also hold a rag saturated with lube around your cable to do this. Best way to do it is to disconnect the cable ends and pull the housings off in order to apply quality lube into the entire length of each. Problem with this is you will most likely affect your derailleur (shifter mechanisms at rear and front sprockets/chainrings) adjustment even if you put the cable back in the exact same place.
If you have a low quality bike you'll need to do maintenance more often if you want the components to last. Department store bikes are not really intended to be ridden off road on a regular basis. Of course you can always support the local bike shop and pay them $50 to do it. If you have an expensive bike it may be worth it but if it's cheapy then you could just replace the bike for the cost of two-three tune ups. I've been told that some bike shops won't even work on Wal-Mart bikes. The first link below covers most of the basics and then some.http://bicycletutor.com/chain-lubrication/http://bicycletutor.com/rust-removal/http://bicycletutor.com/lubricants/http://bicycletutor.com/lube-brake-shift-cables/http://www.parktool.com/