pulling out large dents

I am working on a 93 dodge truck box and it has 3 large dents in it. I have pulled them out with a suction cup, but i don't know how to keep them poped out. If someone was to lean against the box the dent will just pop in again. Paint is not an issue as i am repainting it shortly. Will using a torch to heat the metal help or will it just warp it worse?
Any ideas? Thanks

no, it should help, just dont heat it too much, it sounds like the metal streched, so pulling it out them heating it should help stiffen it up.

I got looking at the body some more and found several small dents/creases that probably caused the dent to pop back in, after i pulled them out the "dent area" would return to its original form, even after pushing it in. COOL! will be priming and painting in a couple of days.