Warpage help.

I just got done widening the left side wheel well on my cutlass. My questions is, what is a good method of shrinking metal to get very minor warpage out? Also are there any tips that anyone knows to avoid this? I welded it up so slowly (metal never got to hot to touch basically) and tacked back and forth when i welded it but it still left a little bit of warpage. Any advice would be awesome as i am about to start welding the other side up tomorrow. thanks