I usually do what others have said here. But I will add this. For the most part, my boots are not sock flexible. That means that for each pair of boots I have, there is a sock combination that works perfect and anything deviating from that combination doesn't. For my winter boots (gortex insulated Army issue) I wear a thin liner with a thick wool outer. If I wear just the thick wool, they hurt, if I wear a thicker inner and the wool, they hurt. If I wear just a thin, they flop around too much. My trail runners are much more sock flexible, but I never wear two socks with them.