There are so many ways, definitely don't pay full price!
Check out Craigslist.org ST LOUIS for sure. If you are taking school age kids, lots of people sell the ones their kids get from school reading programs, and some sell adult (teacher) ones as well. Just make sure they fall within your date.
ALso, know it's a long shot, but on mycokerewards.com they are giving away tons of pairs of tickets all day every day...if you look under their instant win sweepstakes and click on the six flags tickets one, then click "rules" under section 5b, you can get 5 FREE chances to play (without using coke points). I know it seems like a long shot, but last season I did this and won a 4 pack of tickets.
Radio stations sometimes give them away.
Also, there are numerous promotions through Dierbergs for tickets.
Also, season pass holders (like me!) get coupon books where they can get friends in for $15, $10, or even FREE on select dates, some people may be selling these on craigslist as well, and will walk in with you so you can use them.
Have fun! And, one big tip....pack your own food and go back to the car to eat! Tastes SOOOO much better and is a ton cheaper!!
If you have any more questions, feel free to ask! I will help in any way I can!