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I have sheets of steel that I have salvaged that I use for making bases for magnetised miniatures to adhere to.

I need them to be as flat as possible, I have tried hitting them with a hammer, bending them with my hands (sheet is so thin this is easy to do but hard to make flat), and even driving over them with a car (with them wedged between two boards - tends to not flatten them out). I was wondering if anyone else had any suggestions/techniques?

With great difficulty.

The problem is metal has an "elastic" region where it will simply return to it's original shape, so if you have a bent piece, squishing it flat won't take it past the elastic region so it'll just spring back to its original bent shape.

You have to stress the metal past it's elastic point and in to the "plastic" region where it won't return. There's a few ways that it can be done, but without specialised tools it's tough.

You're probably best off googling it and finding some metal working forums talking about it. The first 4 links when you google "flattening sheet metal" might help.

https://www.google.com.au/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=flattening%20sheet%20metal

If the piece is thin enough you might try holding one end in a vice and grabbing the other end with a strong clamp/vice and yanking it, stretching it will stress the surfaces past their elastic point so the metal will hold its shape. But you need to be able to apply enough force to stretch the metal, so it's only going to work for small thin pieces unless you have machinery that can do the stretching for you.

There's reason working sheetmetal is the basis of multiple trades.