Captain Dave: pigoo and toMACsh's answers may seem contradictory, but they are not.
Pigoo is correct that for the most part, the Mac takes care of its own maintenance, and does a lot of that cleanup for you automatically, so you don't really have to worry about it.
OnyX provides an occasional fine tuning and goes a little beyond what the Mac does automatically. I run it about four or fives times a year.
Provided you keep a healthy amount of free space available on your Mac at all times (I don't like to pretend there is a magic exact percentage but at least 10 percent is a good guideline), you have very little you need to do to keep the Mac running smoothly, particularly if you're also making routine backups and installing a healthy amount of RAM for the level of work you do.