I'm concerned less with the power in my own punches at the moment, and so haven't been using the more common drills. Outside of the more obvious stuff [using a weight for some arbitrary number of repetitions or using something to create some pressure, like plyo rope] I've found that one of the best ways to build power has been to work the same punch repeatedly on a double-bag.
Timing has a lot to do with power when we're dealing with the quicker, more technical punches off of the head, and often just focussing on working on getting the timing with the punch right increases the hand speed, which improves the power. One of the things that I've been drilling a lot the last six months has been just trying to increase my technical proficiency and my hand speed, rather than focussing on power. As a result of both, my power has increased a lot.
Video would be good, as has been previously noted.
Sig by Toxic
Barnett - Toquinho -Werdum - "Nurmie"
Z. Gurgel - Morango - Rocha - Tiequan