idk if this makes sense, but do like a sideways tuck... like when you do a 720 really fast, you make ur whole body kinda skinny, and straight up. And then when you get to around 270, stop the "tuck" and try and be fat (doesnt really make sense but w/e), or "less skinny", so u dont overspin. underrotating isnt a big deal, u just gotta turn ur wheel a bit. When i first tried 360's down a 6 set, my problem was overrotating. i would land like a 380 (cuz i was used to spinning fast, like flat), and i ate shit. also, when you are approaching your "obstacle", carve a TINY bit, and like brandonweir said, lift front and then back wheel. Commit.
BTW, took me like 7 tries down the 6 set, just for an idea