pretty much if any of the leavesstart shriveling up and dying, cut em.... but for the most part just any big leaves that are blocking the buds from getting light, trim em back.... unless your growing scrog, then after couple weeks of flowering, trim everything under the screen...
Check out the last page of my grow journal. Low Stress Training (LTS) I am starting my training while in veg, but I am guessing you can do this while in flower as well. It bends the top of the plant over, giving all bud sites underneath a better chance for light/growth. Gluck with it, hope the pics help. Gg