Since humans first cultivated sheep, cultures have made soft sheepskin boots. Their popularity diminished once leather tanning of more durable leather products like cowhide increased. Unlike cowhide, the sheepskin is soft and easily abraded, limiting it to gentle use.
sheepskin boots gained in popularity as a result of World War I and World War II, when they were popular with aviators because of the need to maintain warmth in non-pressurized planes at high altitudes. An exhibit of a WWI aviator's outfit in the Canadian War Museum cites the term "fug" boots. Ugg boots have also been popular with surfers and competitive swimmers since at least the 1960s, for keeping warm while out of the water. Uggs became ubiquitous among Southern California surfers and Southern California downhill skiers, and from there, uggs were popularized by Hollywood into the American mainstream and later the worldwide market.  by JCPugg.com

JCPugg.com advice
There are a couple of things to keep in mind before attempting to clean your ugg boots. Your ugg boots are made from natural merino sheepskin leather. While they are very durable, you need to be mindful of stretching and staining that can occur with certain chemicals. When the leather becomes wet it will be very easy for your uggs to stretch and loose shape. With this in mind, we suggest using slightly warm water to clean your Ugg Classic Boot. After you¡¯ve followed the directions below, the sheepskin will most likely shrink a little, after a few days of normal wear they will stretch back once again. Finally, please know that don't wash them in the washer!

Things You¡¯ll Need:
Sponge
Cold Water
Baking Soda
Paper Towels Or Newspaper
Essential oil
Corn flour
Suede brush or lint-free towel
Leather cleaner and conditioner

How to Clean Ugg Boots
Step1 Use cold water to lightly moisten the outside of the Ugg Classic Boot only. Do not submerge them in water.
Step2 Dilute a cleaner appropriate for leather to form a solution of one part cleaner and one part water.
Step3 Apply solution to a wet sponge and gently clean boots' outer surface. Rinse with clean sponge dipped in cold water.
Step4 Stuff boots with paper towels or newspaper to hold shape and dry out of direct sunlight or direct heat. Direct sunlight or heat will cause shrinkage.
Step5 Brush boots with a suede brush or lint-free towel to restore the nap.
Step6 Use a mixture of 2 tsp. baking soda and 2 tsp. corn flour to deodorize and soften the inside of the Ugg Boots. If you wish, you can add a few drops of an essential oil to create a sweet scent. Shake the mixture into the Ugg Boots and let it stand overnight. Shake the excess out the next day.

Keeping Them Clean
While Ugg Boots are extremely warm during winter, they are best kept inside away from snow or at least warn only in dry conditions. When sheepskin is wet it becomes easily stained and will stretch. Merino sheepskin is extremely durable and if kept dry will last many years. By JCPugg.com