Rebates are a way for companies to move product. If their products aren't selling well, it's costing them. It's also sort of a double-whammy. They expect a lot of customers to slip up on the requirements so they can deny the rebate. I actually think some companies are vague in their instructions so you miss a requirement.
Here's a bit of advice: Don't buy something just because it has a rebate. Think of it as a bonus that you may or may not get.
When I send one in, I do the following:
1) Triple-check I have the required documents (original UPC, dated receipt/invoice, completed rebate form) and that I've followed any special instructions (eg. circle the purchase price on the invoice).
2) I photocopy these documents twice (I also photocopy the addressed envelope for my records). Keep one full set of copies in a folder until I receive the rebate.
3) The other photocopied set gets mailed along with the originals. This is a backup copy for them in case they "lose" something. It also tells them that I have made copies of everything, so they'd better not try to screw with me. I clearly label the second copy "Backup Copy." I also write a quick note on the photocopied UPC that "this is only a copy - see other bundle for the original UPC."
4) I staple each bundle! This is important. Use two staples and put the original UPC on top so it's visible and secured.
5) You'll need two stamps on the envelope with all these documents. If the rebate is worth a lot (I just sent in a $50 rebate for an Antec P182), I will send it as certified mail for about $2.50. They put a sticker on the package with a tracking number. This way I can track the envelope to make sure they receive it. I also get a receipt which shows the postmark date. I keep this in my folder with the photocopies.
I'm covered if they claim I'm missing a document. I have photocopies of everything. I also have the postmark date and proof they received the envelope. Finally, I've included a backup copy so they know I mean business.
If you don't receive the rebate you can complain to the BBB and Attorney General's office. You can also try contacting the reseller (eg. Newegg) and they might help you. Finally, you can post a negative review on
http://www.resellerratings.com. There are a lot of people from newegg on there that didn't receive their rebates. They complained and someone from newegg contacted them (to save face). A lot of the time newegg will work with you to make you happy.