I used to get bumped on a regular basis on the Wednesday before Thanksgiving and the last Friday before Christmas when I was doing a lot of business travel.

For me, early afternoon flights seemed to yield the best chance of getting bumped AND still getting confirmed on a flight later that day. A couple of times I was able to get bumped from flight to flight, picking up multiple vouchers.

Note that United vouchers are considered by most to be the most restrictive and a nuisance to redeem. American's vouchers, on the other hand, were more like cash - usable on any fare, with a "change" voucher given if you had credit left over.

These days though, I hear all of the airlines have changed their bumping rules. Since I haven't done it in a couple of years, I don't know the latest lay of the land.