Can anyone give me some suggestions on returning a board to flat after it has
gone over the wave?

I have a new customer who has been using someone else to solder his boards and
is experiencing warpage after it has been wave soldered.  The board measures 16
cm x 23.5 cm with two (2) 96-pin connectors (.100" spacing) along the 23.5 cm
side of the board.  He said the other shop's solder pot is about 480 degrees and
it is assumed they are running the board with the short side in the grippers so
as to avoid hand-soldering the 96-pin connectors since they hang off the egde of
the board.

Some additional data:
I.C. orientation: 33 run east-west, 11 run north-south, 1 PLL.  There is one
20-mil pitch SMT QFP device on the board.  The QFP is the only SMT device on the
board.

Any ideas on how to avoid the warpage in the first place and when you do have
it, how to eliminate it?

Thanks to all who may contribute.