I think it's a good idea to go back and see what worked and what didn't. This way when you do it again, you can try to go for the things you knew worked before. If you improvise over the same track or song many times, I think this will happen naturally.

In my band, there are some songs where I improvise the solos. Some of the songs we've played often enough that my "improvised" solos really aren't any more. They evolved to a point where I'll basically play them the same each time, but I'll embellish it. That's when it gets fun because I don't worry too much about it...I have ideas formulated already.

I do hear what you are saying though...at some point, it's not really improvisation any more :).