Listen to your bike. Every time that I ride I make a mental list of any maintenance or changes that I want to make before I ride that bike again. That's pretty much all that I do.

A chain, for example, may need to be cleaned and re-lubed after only a 20 or 30 mile rain ride or not require any attention for several hundred miles. When it starts making noise, it's telling me it needs a little attention. Commuters in the Seattle area may find it necessary to replace their rims every year while people like me may never experience significant rim wear. Tires have a way of telling me when it's time for replacement - they get frequent punctures.