If you want to continue wearing your Uggs and keep them in good condition, you have to look after them properly. You probably know how comfortable Uggs are, so you want them to look good as well. A bend mark in the heel is a common problem because of the way the boots move when they're being worn. Although there is no cure, you can prevent this from happening too much.
Buy Ugg boots that fit you properly to avoid bending them too much. Walk around in the Uggs before you buy them, and check the size. If the boots slip or slide up and down your leg they're too big. This may make them slouch and bend. If they are tight, they are too small, which can make them bend.
Stuff your Uggs with newspaper when you're not wearing them. This keeps them upright, so they don't bend at the heel. Scrunch up balls of newspaper and put them inside the boots.
Designate a corner in your wardrobe for your Uggs. Make sure nothing leans against the boots, making them bend over. One solution is to put them on top of a cupboard.
Walk upright in your Uggs and don't run. Overextending your foot may make them bend. Walking slowly will help.