Watch and study shows that incorporate sketch comedy. Besides the aforementioned "Carol Burnett Show," popular sketch comedy shows include "In Living Color," "Saturday Night Live," "Your Show of Shows" and "Mad TV."
Learn to do impressions. Become a caricature. Wear wigs, make-up or a fake moustache. Speak with an accent. Use exaggeration and parody to portray a character.
Strive for originality when it comes to setting. For example, don't use typical or predictable settings. If you're going to do a sketch involving police officers, avoid setting the sketch in a police station if possible.
Practice. Use improvisation when practicing a comedy sketch. Plenty of good material can come out of having the actors improvise during the sketch.
Incorporate physical comedy if possible. Pratfalls and props are great ways to generate laughs when you do a comedy sketch. Sketch comedy shouldn't rely heavily on dialogue.