Some filters can be scrubbed, such as the ceramic core on MSR filters, but the PUR has to be replaced. Fortunately, that isn't a big deal. They cost about $30 online and are a snap to install (www.purwaterfilter.com). If your filter is a few years old but just lightly used, though, I'd leave itthey don't "go bad" just sitting around.
The best thing you can do to ensure long filter life is to filter water that is as clean as possible. Look around for clear water if filtering from ponds or quiet streams, and keep the intake hose away from sand and silt. You want to avoid running currents, too, as moving water can stir up gunk from the riverbed. If the water looks really ugly, then fill a cookpot or other receptacle, let stuff settle down for 30 minutes, then filter it.
A few simple maintenance tricks post-hike or camp will also prolong the life of your filter: heed the manufacturer's cleaning instructions, try the backwash approach to dislodge any crud caught within the filter itself, and sanitize with a diluted bleach solution (unless otherwise recommended) before you stash it away for the next couple of years.