Many common recommendations emphasize the importance of a strong idea to drive the piece when writing a humorous skit. Without a specific viewpoint or worthwhile comedic opinion, a sketch can flail in its momentum or flounder while trying to find punch lines.
The process of writing a comedy skit can vary greatly from one writer to the next; every creative person has her own approach to writing, and many different systems, practices and ideologies are employed with varying degrees of effectiveness and success.
In addition to a strong central concept, a comedy sketch requires rich characters with relatable tics and quirks to humorously propel the storyline. In some sketch comedy scenarios, it's the well-observed characters themselves who serve as the ideas of the pieces. Well-drawn human caricatures skewering a certain demographic, comedic type or human trend is a powerful driving force for comedy.
Similarly, spoofs of celebrities and Hollywood culture in general are solid springboards for comedy sketches. The subject matter's easy, automatic reference points make parodies and send-ups easier to approach for some writers than more esoteric material.
Professionals often recommend watching and, more importantly, reading as many examples of the genre as possible. Classics such as "Saturday Night Live," "Monty Python's Flying Circus," "Kids in the Hall," "SCTV" and "The Carol Burnett Show" offer numerous opportunities to study content and execution.