what temperature setting for how long?

Having been in the record business years ago - the dirty little secret here is that if shrink wrap is applied tightly to the record ( it almost always was because if applied loosely the records tended to tear out the shrink wrap when handled)- this will cause warping - tightly shrink wrap it for multiple decades and it's almost a given.
This was a particularly a problem with the super light "oil shortage" specials of the late 70's. 180 gram records may not guarantee a flat record - but at least you've got a chance.

I've never done records, but being a photographer I've worked extensively with dry mount presses which are intended for mounting and flattening prints. They use heat around and can be adjusted through a range of around 150-275 degrees as I recall. Most important though, and no one has yet mentioned this, is that when removed from heat the object you are trying to make flatter needs to be put under a flat weight while it is cooling down or that change in temperature will cause it to curve even more. At least this is true with paper-based material. We would use a steel platen to weigh down prints and allow them to cool. I don't know if that is as necessary with a plastic record, but regardless, I wonder if anyone's used an old dry-mount press to accomplish the same thing? They are relatively inexpensive on the used market in the size necessary for an LP. There is a manufactured paper specifically for use in a dry mount press called Seal Release Paper which is made to endure repeated heating and never stick to the surface of the print or get stuck on dry mount adhesive. I would suggest using that paper between the records and the steel platen of the press and of the cooling / flattening platen. This is all speculation, mind you - but I'd bet it would be pretty effective.

Hey Marco- IIRC, there have been folks who have used and reported success with dry mount presses (for removing warps). I think that the megabuck "flattener" out there is just a dry mount press turned on end. I have a friend who is a photog who offered to lend me a press and I will report back. I was not aware of the seal release paper and just planned on using a clean sheet of glass on each side of the LP. I plan on trying 170 degrees and moving up in 5 degree increments. If the oven/glass/brick sandwich method works, there is no reason why a dry mount press would not be quicker, easier and more repeatable. Just switch it off and let the LP come back to room temp under pressure. I can't think of a reason why it wouldn't work. Maybe our good buddy Albert will chime in.

A.P.'s too busy making love to his Korean preamps to bother with these forums, unless someone mentions Natalie Portman...now watch, he's bound to show up.

The only reason I'm thinking Dry mount press is that it has the pressure and heat built in. I think it's a good idea to maintain some pressure (hence the weighted platten) as the plastic cools or the rapid temperature change could cause warping in itself, no? I guess if others have had success with the glass sandwich perhaps not. The Seal Release paper is only a theory on my part, but it is designed to not stick to anything and withstand prolonged heating. You're right, if anyone's tried this technique it'd be Albert. Lets see if the Portman technique will lure him in!

I was walking by my computer and swear I heard someone whisper, "Natalie Portman"