For crying out loud, PLOT!
The old adage if you fail to plan, you plan to fail is (usually) true.
When you are performing improvisational comedy you have to have a strong structure in place. For example: We’ll play the props game. So I give you a prop and you have to react to it in a funny way with your comedy partner. Well, you and your partner have decided beforehand that you’re going to perform a western scene where he/she is the robber and you are the sheriff. Then you set up a situation – he/she is robbing the bank (audience) and you show up on the scene to save the day. The scene will end when you somehow kill him/her with the prop. Within that structure you create your scene. “Ideas” are taken from the audience but the show is run and set up (beforehand) by the one running the show. Believe me a lot more work and practice goes into Improv than they would ever lead us on to believe.
Why the long story? My recommendation is to write in the same manner. The more structured your story the easier it is to get a lot of power into your writing. Otherwise, it’s very easy to meander and put down stuff that does not progress the story. Does that mean the structure is law? No. It means that you have confines to work in that will channel the ideas as they come to you about your story. SO…an outline helps, a detailed outline is even better. You don’t have to have all of the specifics (in fact it’s better that you don’t) but you should have a direction that keeps you moving. This also helps you from getting writer’s block or just setting the story aside because you are not excited with where it has gone.
Of course, you can always go back and fix your writing later, but, for me, that’s not as much the fun part :).