What to do with a warped panel?

I am now completely done with the drop front desk I have asked about 20 questions about. It looks great except...
The drop front is about 30"x18" and fits flush in a frame when it is closed. At least it is supposed to fit flush. The panel warped a bit and one corner sticks out. It is no more than an eighth inch, but its not right. I thought about planing it down to fit, but decided it was better the way it was.
The customer didn't even notice the problem, but that is not the issue.
Should I ever be dumb enough again to try to get a large unsupported panel to be in perfect register with a flush frame, what could I do differently?
Using plywood is the obvious solution, but is there any other way prevent the problem? Maybe leave the frame a bit oversized, and plane it to fit the the panel? Naw, that can't be right. Well, any suggestion would be appreciated.
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