How do you stop a peace lily from wilting? when should i prune it?

I don't know if yours is the same variety as mine, but I water them "wet to dry" method, and believe me, when they are dry, they flop quick. Make sure you water them well, and may I suggest an attractive drain pot. You don't want them saturated or too soggy all the time--you WANT the wet to dry. I prune the stems of bad leaves or the bloom stems when they are done, that's about it. Could be your lilly just needs a bigger pot and new soil. When they get too big for the pot, I take them outside, saw them in half, and then I have 2 lillies, or 3.