I got back a few weeks ago from a trip in the Winds. Two of the days up there we got dumped on. I find that having a tarp under your tent that you roll up around the edges literally simulates trenching. Well, that is to say that no water makes it inside. You have to carry a little bit heavier tarp though, as it's larger than the area of your tent. I'll try to make an illustration here.

This is a cutaway of half the tent, you can see how the rolled up tarp sheds water and lets none inside. You just have to make sure you've done a darn good job of it or else you'll be a wet one in the morning.