A gunky, kinked or rusty chain can slow down your pedalling efficiency and cause excess wear on your drive train. Lubricating your bike frequently can help keep your chain happy (avoid using WD40 since it leaves your chain dry). For best results, clean your chain, gears, chain rings and derailleurs with degreaser then lubricate.
A rusty bike is an unhappy bike
Read the full post: The ABCs of your Bike
(cross-posted to the Bike to Work Blog)