Very good advice -- kudos:)
Personally, I've only had a problem once, out of about 30 or 40 mail-in rebates total, but that's because I do keep photocopies of EVERYTHING, and I try to be as forward-helpful as possible (i.e., as well as filling their forms with font size 6 handwriting [who came up with boxes that small???], I'll also type in my name/address/details in big letters, I staple everything nicely together so its not lost in the envelope, I circle the relevant info on the receipt, etc)
One time The Learning Company (Compton's line of products) sent me a refusal... this was, of course (Murphy is a God) the only time I forgot to make copy of UPC (although I had copy of receipt). So I sent them the jewel case insert of my CD with copy of receipt and begged forgivness (although I guess they received UPC the first time around...), and I received it months later...
Mind you, they usually take at least 2 months longer than they claim... I just don't let that bother me... I buy stuff on Future Shop Card, I don't have to pay it for almost 4 months anyway:)