Disguise is the art of changing you appearance without making it appear that you have done so.
If you think that a wig and fake mustache are a good disguise you have allot to learn. People often remember faces by only a few features. A big nose, style of glasses, smile or facial tic all are features we use to memorize the faces of people around us. A disguise is designed to give people something else to notice or even something that was not there before. What is important to remember is that we do not want the disguise to be so interesting that people will remember it even if it effectively covers your identity. A bad disguise is worse than no disguise!
An easy way to throw people off is to find some thick rim safety glasses or low power reading glases (don't impair your vision), don't use something too retro that stands out, they should be passable as real glasses. If you always wear glasses you should have a few weeks supply of contacts and solution, many manufacturers will mail you free samples.
If you need a short term disguise, the old IRA standby of aviator sunglasses, a bandanna worn desperado style over the mouth and nose and a hat makes you nearly impossible to identify, but this is the sort of thing you wouldn't want to be wearing in public. Taking the bandanna off leaves you with a good disguise, but you can still be recognized by advanced surveillance just by your mouth and nose.
Dye and Hairstyle
A bad dye or bleach job will stick out like a sore thumb, the same with a bad haircut .
An easy haircut is to use the shorter trimmer tips on an electrical beard trimmer to give a close cut for someone with long hair, an even better one is to shave it totally off, which can greatly alter your appearance. A bad dye job is one that does not match facial hair and eyebrows, also it is obvious as it grows out.
Shaving and a preppy haircut are surprisingly effective disguise when combined with a new wardrobe.
If punk is your style switching to button shirts and khaki or a suit is a huge disguise. Don't do the nice clothing thing if you don't know how to act the part, that will stand out.
When dressed up be sure to walk and move in a purposeful way with your chin up, a mobile phone you can constantly pretend to jabber on is a good prop to make people feel they need to look away, don't be annoying or loud though.
Try to wear the plainest clothing possible. Logos, bright, unusual colors and odd styles of clothing will make you easier to identify. Jeans, solid colored shoes and a solid colored shirt make you very hard to spot when worn in conjunction with a hat and sunglasses.
Whatever your race or ethnicity all good hard working Amerikans try to be WASP's. Remember this and suppress your natural in your face humor if you are trying to stay off the radar. Every morning repeat this mantra for ten minutes "I am a good hard working WASP". Looking shifty, stressed, or worried will stick out more than your appearance even if your face is all over the papers and TV. Be cool...
If you can find a good acting class they will actually teach movement, posture and attitude modification, advanced classes will even teach how to fake an accent properly.
Piercings and Tattoos
All forms of piercing, body mods, and tattoos must be covered up. Wool hats can cover ears and eyebrows, bandages long sleeves and gloves can all be used to cover up hands.
A bandaid plaster or some acne cover-up on your forehead is sure to distract the casual interaction like at a bank or convenience store. Don't get too elaborate with distraction as it might begin to draw attention.
Hard to Change features
The following are very difficult to fully disguise:
Race: There are medication that will darken the skin, spray or natural tanning may also work but this will not alter other race specific facial features.
Sex: hard to fake, going for the androgynous frumpy look might work.
Age: difficult to fake even with stage makeup, up close it often looks fake
Height: good posture or slouching will alter apparent height.
Head Shape: Some medications can cause swelling in the face but be careful of side affects.
Facial Features: Drastic measures include the home nose job which involves a "surgeon' who knows how to give a nice punch and then reset the cartilage, this gives minimal alteration of the face long term but leaves you with two black eyes for a week or two.
Drastic changes in diet can cause drastic changes in appearance both weight gain and loss but this takes time as does building a muscular appearance.