My Getaway had a lot of seriously warped plastic bits. I looked for the best way to flatten them, and found this thread:
http://pinside.com/pinball/forum/topic/playfield-plastics
as well as some others. They all basically boiled down to two techniques:
1) Quick but scary: Put them in your oven at 225F and wait for them to just start to flatten then remove heat and press them down (but if you wait a few seconds longer to remove heat they might bubble and be ruined). There were also heater/hair dryer variations of this as well.
2) Slow but safe (but only possible in hot climates): Put them between two sheets of glass in the back of your pickup on a day when it's hotter than 100F outside and wait 2-8 hours. No tricky timing, no chance of harming them...

So the choice was basically either really risky (practice with some pieces you can afford to mess up) or safe but impractical (not everyone has a 100F day whenever they need to flatten plastic).

Well it was obvious to me that you should be able to combine these methods to get the best of both techniques. So I got two sheets of glass cut to fit in my oven. I put the (cleaned) warped plastic between the (clean) sheets of glass and let the oven heat up to 150F. After an hour or so I put a weight on top of the top sheet to press them together. An hour or so after that I turned the oven off. Once they cooled down I had pretty flat plastic (the slingshot plastic had a section that was really bent that I never got perfectly flat - I think some combination of more weight/longer time/higher temp would have fixed that, but I was happy enough with the results that I didn't pursue it).

I started experimenting with an oven temp of 120F - that went OK but I wanted to see how high I could go without any harm. I ended up going to 150F, stopped there because it was working fine and I didn't see any point in going higher. I didn't pay attention to time - put them in the oven, go to the next step about an hour later. I don't think there was any way I could have ruined the plastic at 150F.

Here are some pics:
Big Piece Before:
2. Big Before.jpg

Slingshot Before:
1. Little Before.jpg

In Oven:
3. In Oven.jpg

Big Piece After:

Slingshot After:
4. Little After.jpg

This could probably be optimized to go faster - you probably don't need an hour between steps, and you might be able to add weight/pressure at the start (I wanted to give the plastic room to breath/expand out as it flattened) - but I wasn't in a big hurry.

Hope this helps some of you guys who have helped me so much in the past!

OK, so I guess there originally were THREE distinctly different approaches to flattening...I'll amend my original post.

Using Bake is fine. Broil (I think) means MAXIMUM HEAT so I'd stay away from that one. Airflow (like in a convection oven) might make the plastic get to 150F a bit sooner, but this is the no-hurry process, so I wouldn't worry about it. I did it in a convection oven but I left the fan off (just like a normal oven).

The thing you're trying to do is replicate the temperature between two sheets of glass left in the sun on a really hot day. Let everything settle to that high temp for a while, the plastic relaxes and the pressure forces it flat.

I think the main thing to look out for is make sure your oven temp goes that low. My toaster oven (for example) doesn't go below 200F.

I just went to my local hardware store, gave them the dimensions (I think I went with 12" x 18") and they cut two pieces. It's not the rounded edge, tempered glass we're used to in pinball so I was more careful with it, but it worked fine...

Of course! IKEA to the rescue once again!

Good question. No problem at all the one time I did it. Glass plates were perfectly clear after, plastics looked good as new (or at least as good as before the flattening).