+1 for heat gun. Not that the oven method wouldn't work fine - I just never tried it.

In my experience, I heat them up til they flatten themselves out good on a piece of glass then sandwich them under heavy stuff until cool. Also, in most cases for me I had to do repeat once or twice more for them to actually be flattened, but it's pretty safe and it eventually worked.

You do have to be careful to move the heat gun around and heat evenly. Linger too long in one spot and it WILL start to scorch the edges. Just keep it moving and heat evenly and it's pretty safe.