I've done it by finding a screwdriver that will just barely fit. Usually the hex portion is not completely rounded out and if you can find a corner to catch the bit it is often enough. Another possibility is to get the next larger size allen wrench and simply drive it in enough to get a grip/
Yeah, torx is what I always go to. Also consider that allen wrenches are both English and Metric; maybe find the next size in another measurement system. I like Charlie's idea, I'm going to sock that one away for a rainy day.