Whatever has happened in a scene so far, add more detail to it.
An improv scene is little more than each person adding information to what the previous person has just added. In this way, a reality grows quickly and spontaneously. Every little bit of information provides thousands of "hooks" that your partner can use to keep on building the reality.
For example, your scene partner has just said, "My mouse button is flaky." What information could you add to that?
You could mention the computer that the mouse is hooked up to: "Wow, an Acer 486. I wish they'd give me such a nice computer." Now your partner might say, "I won it in a scholarship from Acer, the happy PC people."
You could mention the computer program that the person is running. "Photoshop...hmm, version 5. That's, like, way out of date." Now your partner might say, "Oh, now that you're working for Adobe, could you get me one of those...educational discounts?"
You could describe the brand name of the mouse: "A Logitech mouse? A Logitech mouse has a flaky mouse button? This is unheard of!" Now your partner might say, "I designed this mouse. It's my responsibility. Now, the hara-kiri sword, if you please, time is wasting." That could lead to the sword being flaky. Then you announce that you designed the flaky hara-kiri sword and will have to do the honorable thing...
You could describe the mouse pad. You could be the repairman who's come to fix the mouse. You could be a trackball salesman. Adding information is trivially easy because just about anything that the previous information triggers in your mind is adding information. And yet adding information is pretty much all you have to do to make a wild and wonderful improv scene that is so spectacularly imaginative, it seems like mere mortals couldn't have thought it up. (Shh, don't tell anyone that the magic trick is this easy.) This is so very very boring.
See also: Context, Be Concrete, Yes And