A lot depends on how and where you ride, but assuming that you're riding single track with your friends and that you're getting dirt and grit on the bike, then you'll need to inspect it and give it a wipe down after each ride. Pay particular attention to the shifting and braking, and make certain that they (the mechanisms) are well tuned and free of crud. If the chain is gritty, it will wear faster. Inspect, lube and wipe.

Also, if you have rim brakes, then you'll have to keep on top of the wheel true. If disc, not so much.

Don't direct pressurized water toward any bearings, but you probably know that.