I have been altering my purchased (and some home-sewn) blouses lately with elastic in the back. It's super easy, adds some cute ruching, and makes the fit a little bit more formal—which is nice, I think. Especially when I am not in the mood to tuck in my blouse, which is often. Here's the front of the blouse, on the left, and then the altered back, which used to just lie flat, but now has two rows of small gathers between the back darts.
I made a little tutorial on how to do this. I'm a little spacey sounding in this and seem to be forgetting words. You know how it goes.
**update: I swapped in new photos so you can actually see the blouse on. It actually looks exactly the same in front as it did pre-altering—just the back looks different. Of course, don't try this technique if your blouse is tight to begin with.