Making myself a rig for a table top drill I ran into a problem with warping of a rather thick mild steel plate welded to heavy peace of tube. I wasn't easy to find a solution (but many with a similar problem) and I got many a weird advice on the way. Eventually I got it straightened, and I thought I might shared my findings.
Thanks to Hotrodder's tip in a UK forum I found the knowledge and the courage to do what I intended to in the first place.

I used O/A to heat the plate on the opposite side of the welding, using a very narrow flame and trying not to melt the iron (for appearances sake). Discolouration is easier to grind away than melted scars.
I then left it to cool and were pleased that it had helped. I did several turns, and eventually the plate was almost flat again.
Here are some pictures to illustrate the task.

Click image for larger version.  Name: Before.jpg  Views: 349  Size: 66.4 KB  ID: 5700Click image for larger version.  Name: after.jpg  Views: 361  Size: 75.7 KB  ID: 5701Click image for larger version.  Name: Topview.jpg  Views: 370  Size: 90.7 KB  ID: 5702