Leather spray and clean them ASAP when you soil them. They stain easily so there's not that much else you can do. Salt is a boot killer so just be sure to clean them every time you wear them out in the salt.
there are ugg cleaning kits you can buy at most shoe stores that sell uggs. I already had a multi purpose spray protectant at home though so I didn't buy it.
I spray them every so often in the winter, and if they get dirty I wash them with a soft cloth in cold water with a mild soap. let them dry naturally and stuff them with paper towels to keep the shape and then use a stiff cloth to brush them
I just bought another pair of Uggs last night (black snakeskin from Nordys) and bought more spray. I think I am going to spray them 3 - 4 times before taking them out in the winter snow and sleet. Luckily they are black so the dirt won't show. Buying dark brown and black is one way to ensure that they stay clean! haha