-Tire pressure. Check it every week. This is the most important thing you can do.
-Wipe down your bike to keep it clean, especially the drive train (I like to clean the cassette by sticking a rag between the gear every few weeks).
-Depending on your conditions, you should also take apart the jockey wheels every 3 months to a year, then wipe them out and smear the inside with grease (not lube). I took mine apart after a year and they were filled with grime. The cleaning also got rid of an annoying squeak.
-Your chain will need to be replaced more frequently. A good rule of thumb is every 1500-2500 miles, but your LBS can measure the chain to see if it has stretched enough to be replaced (or you can buy the tool to measure it yourself).
-Watch the break pads.