Keep an eye on them for play. Occasionally grab a handfull or wheel and see if you can detect any sideways play (in the pivots, not wheel). If there is some, new bearings time. Similarly hold the wheel down, and pull the saddle up, if you can feel play/knocking it's probably new shock bushes time. Finally, it's worth occasionally whipping the shock out. Move the rear sus, it should move smoothly through its range (a little stiff is ok, but if it's notchy, new bearings time).
Replacing bearings as they start to wear is a good idea. Other wise bad wear can cause them to seize and rotate in the frame, knackering the mounts.
Oh, and of course checking bolts are tight occasionally is always worthwhile in rear sus as with anything else on your bike.