It might sound a little odd, but one way to care for sweaty feet in your UGG boots is to use antiperspirant. Deodorant for your feet works just like it does in your pits, except it's stronger. You also can try UGG boots on sale. If your foot sweating is really bad, you can see a doctor about getting a prescription deodorant.
Whether you wear socks or not with your UGGs, you'll want to spray down the insides with an anti-fungal spray at least once a month. The sheepskin lining of your UGGs makes them an ideal environment for bacterial and fungal growth, especially if they are moist from sweat. Fungus can make your feet unsightly, ruining sandal season. And it's bacteria, not sweat itself, that makes your feet and shoes smell. So if the insides of your UGGs smell like cheese that's gone bad, you'll know you've got bacteria growing in there.
Make sure your feet are good and dry before pulling on your UGGs. In addition to towel drying, you might want to give your feet a blast from your hair dryer just to make sure. A surefire way to cut down on foot sweating in your UGGS on sale is not to wear them so much. When it's warm outside, wear sandals as much as possible to air your feet out, and wear your UGGs sparingly -- maybe just for an occasional night out on the town when you want to look fab. In colder months, give your feet a break and take off your UGGs when you are at home.