welding technique question

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Doing some sheetmetal work on an old mustang.  Learned allot about  heating, warping, blow-thru, etc.  I've gotten better, especially my confidence level, but there's one area that I find troublesome; butt welds.  Both sides of the sheetmetal has a tendancy to pull up through shrinking and contraction after it cools.  My welding is good, good penatration, minimal heat zone, but like clockwork the two pieces will warp just a little immediately adjacent the weld.   Often, I can't get behind the pieces to provide support or an opposing weld to anticipate shrinking.  I recently set a door gap on a quarter panel with a relief cut, welded, ground, looked great!  Then......    Sure, less than a 1/16" of bondo will cover the prob, but how do you prevent it in the beginning?
Reply:If you are using mig, slow down and stagger your weld. instead of doing a long pass, do  small tack about 1/4 inch and not allow the metal to get to hot.
Reply:high localized heat input, skip welding and tacking can help.  So can back welding.  The other big thing is preheating the whole area so there's no significant difference in temperature.