The popularity of UGG boots has really taken off in the last decade. UGG boots are made of sheepskin so need to cleaned and protected like any suede boot.
The best plan is to waterproof to protect them as best as possible from the elements.
The Meltonian Water and Stain Protector. is a great product because it will protect the sheepskin or suede without darkening the upper but, as with all leather care products, always test on an inconspicuous area just to be sure. If there is surface dirt or dust brush with a suede brush or nylon brush first and then spray upper evenly. If you can spray it outside that would be best and let them dry throughly before wearing.
If your UGGs need to be cleaned you can use the Lincoln EZ Cleaner.. It is a mild leather and suede shampoo that can be diluted with warm water.
First brush the boots with a suede brush to remove surface dirt, use a suede eraser for any stains. Wet down the boots with a wet cloth, shampoo with the EZ cleaner on a sponge using a circular motion to clean the boot doing the entire upper. When finished cleaning, use the wet cloth again to rinse off the cleaning solution with clean water. Stuff boots with newspaper or a bootshaper to keep their shape and allow to dry away from direct heat. When dry, brush again gently with the suede brush and this is when you can waterproof them again.