Re-using filter cartridges is very dangerous to the pool stores' revenue stream! Don't do it, they NEED the money!

All jokes aside, lots of folks do exactly what you're doing. As SunnyOptimism said, acid wash as needed (make sure to degrease with dishsoap before an acid wash). Replace the filters when they get holes in them - usually long after the bands start to break off.

Pooldoc wrote up detailed instructions on cleaning and maintaining cartridge filters here:
http://www.poolforum.com/pf2/showthr...ut-Damaging-It!