Keep photcopies of all submitted rebate documents and send your submission by registered mail. Someone will have to sign for it when it is received. You'll be surprised how fast their tune will change when they tell you they never received your rebate mail and you can prove they did... 
Have all your rebates sent via registered mail been accepted? I heard that some rebate companies won't sign for registered mail. I never tried it myself. Thanks.
Mario
I've never tried it either, I hang out alot on the redflagdeals forums (canadian deals site) and many people there send them in that way and highly recommend it...
I do, however, go through my rebate forms with a fine toothed comb and make absolutely, positively sure I have done everything it asked "to the letter". I also always keep copies of everything I mail off. I've never not gotten a rebate I applied for...