The best Washmitt IMHO that is cost effective, and gentle on your paintwork etc is the Eurow Mitt, available
Here at CleanYourCar. I had a couple that lasted me a good 12 months, being used 2-3 times a week.
The best way to care for them, is to always keep them wet, and not let them dry out/shrink, so I always tend to let them soak in water. I pop them in the Washing Machine, with no detergent at 30 degrees to clean them, you can tumble dry them on a 'Cool' cycle if you need to dry them for any reason.
HTH,
Gaz