Write for your audience. For example, a small-town audience may not be the best audience for a skit about living in New York City. Knowing your audience is the first order of business when creating a successful skit.
Locate appropriate props and costumes. These don't have to be expensive. Search thrift stores and garage sales for big discounts.
Practice transitions. Does your skit come before or after another skit in a program? Knowing how you'll get on and off stage can make the difference between a skit that looks amateurish and one that looks professional.
Rehearse until you know the script by heart. Practice not only the lines, but the movements. Only when you can say your lines without thinking can you begin to experiment with physical comedy and characterization.
Hold a dress rehearsal. There is no better way of learning what works and what doesn't than to rehearse your skit in front of an audience before the show opens.