Ask the audience for input. This a foundational technique for many types of improv skits. At various points in the skit, ask the audience for ideas. For example, you might say, "Here are our characters, Joe and Susan. They are enjoying each others' company where? Any ideas for the setting of this skit?" The audience members can then voice ideas, and you and the acting team must choose one.
Ask rhetorical question directed toward the audience. This doesn't demand a verbal response, but it does focus an idea toward audience members personally. For example, you might say, "Don't you hate it when you can't find your keys?!"
Assign a performance part to the audience as a whole or to sections of the audience. For example, you might instruct audience members to make an animal sound such as a "moo" whenever you say a certain word or give a hand signal. This keeps the audience focused because they must watch for their cues.
Ask for volunteers. Include a role in the skit that doesn't require preparation and ask the audience members to raise their hands if they want to perform. Select one or more members of the audience to join you on stage.
Distribute small slips of paper and pens or pencils to the audience. Ask them to write a brief idea for a skit. Another option is to pause in the middle of a skit and have them write a brief summary of how it should end. Collect the responses and randomly draw one out of a container. Construct the skit around the suggestion.