Finsfan, I can provide you with a method that works like a charm. Although it's a little extensive, you will absolutely have a FLAT record when you're done. I have a 2" thick maple chopping block in my kitchen. I also own a piece of granite that I, at one time, used for a turntable base. It weighs about 40 lbs. The procedure is as follows:
I put the granite slab in the oven @ 175 degrees for approx. 1 hour. I place the warped record in a rice sleeve, on the chopping block with a piece of "oaktag" paper on each side of the record, (in it's sleeve). I place the preheated granite on top of the record and allow it to return to room temperature (usually overnight).
I have used this process on records that were significantly warped. Every time, they come out dead flat. If you own the above mentioned items, it's a really easy and extremely effective method. If you have an extremely warped record, it may return to a flat condition, but the grooves may have suffered from a loss of concentricity, and possibly may now be physically distorted. Then you're screwed.
Best of luck!