I know you asked for just one, Kel, but I can't decide between two. At one point in labour, the warm shower/bath was absolutely the best, especially with the handheld shower head, so I could direct it over my belly, or wherever the pain was worst.
Later, controlled breathing was best, along with carefully thinking about the contraction as a wave. It would wash in, cover me, and wash out again. Remembering that it WAS going to wash out and that I COULD make it through was essential. It kept me calm for my second birth. (with my first, I was pretty much panicking from the pain at that point. Should have had a doula. Didn't know that then. ) Second birth went so much better partly because of that. (also because it simply was a little easier, being a second birth.)

HTH!