I think they frost up if they have to work really hard to stay cold, ie, if they are near a window, the stove, or if the seals don't work properly. Make sure none of these things happen if you can (I know,its a bit hard to change the position of a fridge in a kitchen), and keep the seals clean. If the seals have had it, you can get replacement ones, and I think it can be a a DIY job.

Also, the fuller the freezer is (or fridge) the less work the motor has to do to keep it cool. I have heard of stuffing scrunched up balls of paper in 4wd eskies as they get emptied to keep them working efficiently.