the best day is wednesday. The way to do it is figure out the ride you want to go on the most, which will most likely be the newest one, or the one with the longest wait. just make sure you're there when they open and run to that ride. then there's virtually no wait. Another thing I recommend is the Flash Pass. you pay something like $15 or $20 for four tickets, and you get to go to the front of the line, no waiting, for four rides. Some of the Six FLags have implemented the Qbot, which is somewhat like the flashpass, only instead of paper tickets you get a pager. The qbot figures how long the wait might be, and pages you in that time period so you can do other things instead of wait in line. I like the qbot better because it's unlimited.
The catch, though: if it's a non busy day like a wednesday, they might not offer the flash pass/qbot.