When I crew in a race, I most often run the main and jib, while the skipper runs the tiller and traveler.
I trap out first, and if there are double-trap conditions we usually overlap our legs to keep the weight together. This has the benefit of being able to brace against each other, or hang onto each other, if absolutely needed. I really prefer crewing in a race, I'm much better at it.

If I skipper in a race, it usually means that nobody needed crew, and the person I have crewing for is on the boat because I told them we were just going out for a sail and they have no idea what's going on. I give them brief instruction on running the jib and trapezing and pretty much just tell them what to do and where to be during the race. I run the tiller, main, and traveler (my mainsheet is also my traveler line). I still let the crew trapeze first, but on my boat there are foot-loops (garden hose and old halyard version) to help keep you in place. I try not to ever let them know we're actually racing, or they would start asking why I'm putting around behind everyone else. I really don't like playing skipper in a race, since I don't have much clue about where I'm going half the time.