I'm going to start by talking about endings. Those darn endings. The hardest thing to write - whether it's a play, film, novel or improv scene. "I didn't like the ending," people will argue. "I wanted a happier ending." And then there are those who love the open endings. That makes it "our" ending that we can write in our own heads. Because believe it or not, we are all writers - of our own life. Life is the greatest story of all - because it's the only one that will keep you guessing until the very end.
Improv is instant writing. It's just writing without tools. That is to say, the only tools are your voice, face and body. So finding an ending for an improv scene is just as challenging as finding one for a written story. Except the deadline is a lot tighter. Your audience is waiting right there. "Come on! Make us laugh! Satisfy us with a decent ending, wouldja? And don't make us lose interest while we're waiting!"
One of my favorite improv games is called "First Line, Last Line". In it we ask the audience to write down random phrases/quotes/lyrics on pieces of paper which we later draw out of a bucket. Two actors perform the scene, each reading out their respective line. They then must begin and end the scene with whatever is on those pieces of paper. The audience loves it because they are involved, and most of all because they don't really think we can make it work. But we always do. Point A to Point B. Birth to Death. We fill in the parts in the middle. We don't know how we're going to do it, and it's often not how we planned, but we all get to some ending.
Creativity, like emotions, doesn't like deadlines. As a former advertising copywriter, I know this only too well. You can try to create, and it will be fine, perhaps even good, but it won't be phenomenal. The best creativity outpouring comes when it comes. I don't know where it comes from. But I do know that you can't force it. It doesn't have to come from sitting down at the computer and staring at a blank screen. It can come from doing the dishes, or waiting for a tram, or in the middle of a seemingly trivial conversation. It doesn't like to be told "when" or "where" or even "how". IT finds US. It talks to us. But you have to be aware. You have to follow the improv rule of "Listen, Watch and Concentrate". Some call it the still, small voice. Some call it intuition. Whatever you call it, make sure you have it turned on high when you want to create.
And most of all, give it time. Don't give it a deadline. It takes as long as it takes.