Often when I'm finished sewing something I go back and shape it up a bit with my iron. This is especially helpful with the Box Wallet, which I'll use as the example:
1. Stuff the wallet with paper (plain grocery sacks cut into smaller pieces work great).
2. Warm up iron
3. Firmly, but gently, press the edges against the hot iron for 5-10 seconds. Do the top, bottom and sides one by one.
4. Then press the front of the flap and lay it face down to cool off and stiffen into it's new and improved shape!