One of the fastest things to wear out and need replacing is your drivetrain. Fortunately, this doesn't have to be the case. Biggest contributor to a worn out chain, derailer or chain ring is poor shifting (assuming you wash your bike once a year).

Proper shifting consists of three things: Preparation, Power and Position.

Preparation - Anticipate the gear.
When you are coming up to a hill, shift early. Try to maintain your momentum and shift up through your gears to stay in the optimum gear to maintain your speed. Waiting till the last minute to shift over 5 or 6 rings will kill your momentum and require more energy than you want to spend or your chain and rings want to feel. Shift one ring at a time and try to maintain AT LEAST the same pedal stroke speed throughout the hill climb. If you can slightly increase your stroke speed, great.

Power - Put less power into your stroke while shifting.
If you've failed to prepare for that hill and you find yourself increasing your power in order to maintain speed, try giving EXTRA power right before changing gears. Put all your power into a single downstroke than quickly shift while pedaling lightly and continuing on your momentum from the first stroke. Once the chain has completed the move, repeat the process. Reducing the amount of "power strokes" during shifting will add years to your drivetrain (literally).

Position - Keep the chain parallel to the frame.
Although not one of the biggest contributors, your chain position can also be important. Just because you have a 27 speed bike means you should be using all speeds. Having your chain in the smallest chain ring (granny gear) and the smallest cog on the rear cassette ends up stretching the chain more than you need too and causes excessive rubbing and pulling on the ring and cogs teeth. You can easily switch to your middle chain ring and move up a cog or two in the back to get essentially the same performance.

If all else fails and you find yourself unprepared, pedaling with too much power while changing to unparallel gears . . .stop, get off the bike, shift and lift the rear wheel while pedaling to get into an easy gear to start off again. Your bike and muscles will thank you.