I don't know how to practice the art of disguise. Surprisingly, enough the Wiki StealThisWiki (purporting to be an collaborative continuation of Steal This Book [online version]) has some not so terrible (although pretty trite mostly) advise. Other that that the internet seems unexpectedly bare. I really hope that this is because webpages on disguise are hidden well and not because they're not there at all. I have found a couple of books that I am currently reading and will summarize for you later.
In the mean time, let me introduce you to another person who is famous for being masters of disguise:
Emma Edmonds - Edmonds was born 150 years ago in 1841. When her father tried to force her to marry against her will, she left home. Worrying that her Canadian father would find her, she fled to the United States. And what does a woman in 1841 do in the United States? She enlists to fight in the Civil War as Mr. Franklin Flint Thompson. Apparently, physical examinations weren't common and so she was able to disguise herself as a man. Apparently, she is one of approximately 400 women to sucessfully enlist in the army during the Civil War.
Not only was Emma able to pretend to be Frank for several years in the army, Frank eventually became a spy for the Union. For her first mission to the Confederate front she disguised her white-self as a a black man whom she named Cuff. Shaving her head and donning a wig and coloring her skin by using silver nitrate. In this disguise (which was apparently good enough to fool her superior, she was able to spend two days on the Confederate side, long enough to gain important information about the Confederates.
About two months later, Emma disguised as Frank disguised as a fat Irish peddler woman crossed the enemy line again to sell apples and soap and steal secrets. Apparently, she had some sort of adventure getting back to the Union army since she had to steal a horse and was wounded in her escape.
Emma continued to use Cuff as her disguise but also developed a black mammy laundress. Once while cleaning an officer's coat, she found a bundle of official documents in one of the pockets full of important information. She was also involved in setting up a spy network in Louisville disguised as southerner Charles Mayberry.
She was able to continue with her amazing disguises until she caught malaria. As long as she wanted to keep her secret identy, she could not admit her alias Frank to the hospital. So she left camp and her alias behind and checked herself into a private hospital. Unfortunately, this made her a deserter. Her life as a professional spy was done.
She wrote memoirs and eventually admitted that Emma Edmonds and Frank Thompson were one and the same. She fought for (and eventually won) an honorable discharge (as Emma) and got her veteran's pension through a special act of Congress. What a crazy life. Amazing. Most of this information is from Wikipedia or civilwarhome.com