Perhaps you'd like to get a job as an international spy with the Central Intelligence Agency. You might want to do your research and due diligence on what that really means before submitting an application. The glitz and glamour of film and television shows lend fantasy elements to the concept of disguise, but some people really can become masters at this art. They are typically those who study for years in theatrical departments of major universities. Make-up artists and costume designers are the people who often bring alive the characters of film and television. To master disguise, you will need to invest considerable time, finances and maybe even some high-tech gadgetry.
Train in dialects and learn languages that appeal to you. Become fluent enough to make easy conversation. Study with a voice teacher to learn how to alter your voice to upper and lower ranges. You may need a voice device to help you. The voice is often overlooked by people who dress up and think they can fool family and friends. The voice is the first giveaway -- when you can disguise your voice consistently, enough to fool even audio recording comparisons -- then you can add the outer layers of costume and make-up.
Attend a prominent college or university known for its theatre department. Find the professors with awards and kudos and study with them. Take courses in make-up and hair design, including wigs, ageing, racial, ethnic and effects. Practice with hair dyes, cuts and styles in combination with various make-up themes and costuming. Master make-up arts and you're on your way. Be careful with some of the toxic forms, though, as they can cause skin irritations and rashes -- you'll be taught this through reputable schools.
Study costume design and implement the designs you create. Learn about layering, padding, thinning, elongating and other factors that give the illusion that a person is taller, shorter, heavier, etc. Study costuming history and current fashions. It doesn't bode well to show up in Paris in a 1940s steelworker outfit that makes you stand out against the well-dressed citizens walking about in 2011.
Take acting classes and perform live on many types of stages for different theatre companies. Your talent will escalate you into better roles. These roles provide you the necessary talents to "become" different people.
Practice your knowledge and talents by combining your voice, make-up, hair, voice and clothing, then visit friends and family to test your abilities. Once you have fooled all of them (not just a few), continue to your job and work outward into the community at large. When you have convinced a visiting Irish author or a Kenyan drummer that you are from the same clan or tribe, you are getting closer to the goal of mastery.
You could get a government job that requires disguises, but you will still have to train at length to become a master. Governments have high-tech, state-of-the-art equipment that can accelerate certain aspects of your goal, but to get such a job will require a number of other skills as well. Disguises for Halloween, concerts, programs and events can be great fun to create, but remember who you really are under there and where to draw the line between having a playful good time and getting seriously sidetracked.