Any vehicle you own is going to need maintenance to keep performing reliably. If you would like your bicycle to not leave you stranded out on the trail, then you need to keep it in good working condition. If you love your bike, then your bike will love you back.
The majority of people who purchase and ride a bike do want to keep their bike in good operating condition, but they do not know where to start. This list should give everyone a good idea of what needs to be done, and when. If you ride in the rain and / or mud often, or put a lot of extra mileage on, then of course you will need to do this maintenance more often. On the other side of things, if you are a more casual rider, you obviously can use a more relaxed maintenance schedule
Before you go on every ride, you should check the air pressure of your tires, check your brake cables and pads, check your cranks for slop or play, and make sure your quick release hubs (if you have them) are set firm, but not overly tight.
After every ride you should check your tires for glass bits, nails, thorns, cuts, or sharp gravel. Make sure your wheels are true, and roll straight, with no wobble. Clean the various mechanicals as necessary, such as the derailleurs, and brake arms.
Once a week, or every 200 miles (whichever comes first) you should lubricate your chain.
Once a month you should do a full cleaning of your bike, including the drivetrain if need be. Be sure to inspect the chain and your freewheel. Inspect and lubricate the pedals, style cleats, brake levers and all of your cables. Check for looseness in screws and bolts throughout your bike.
Every Three Months
Wax your bike because shiny and clean bikes go faster as well as farther. Inspect the frame and the fork for cracks or bulges in the paint that may indicate part damage. Visually inspect the entire bike looking for bent or broken components.
Every Six Months
Every six months you are going to want to inspect as well as readjust the bearings in the headset, the hubs, the pedals and in the bottom bracket. Some sealed cartridge bearings are incapable of being adjusted and can only be replaced, so only do this if it is feasible to.
Every year you are going to want to disassemble and overhaul the entire bike. You should be replacing all of the bearings if it is possible for you to do so. You should remove the brake cables and shift cables and replace them if it is necessary for you to do so. This should definitely be performed when you hit 6000 miles if you are riding more than that many miles every year. If you are a commuter that rides in the rain or if you get dirty in mountain biking then you are going to want to overhaul your entire bike more often.
Before you take any important journey with your bike you should have a professional go over your bike to double check all of the adjustments so that there are no problems that spoil your excellent ride.