Being my cheap @ss self I do a lot of rebates. Last year ('03) I did exceptionally well, this year was marginal.
A few pointers when doing them.
1) submit the rebate as soon as you have the item
2) print out the forms and record the URL where the form is located on the day you make the purchase
3) cut out the UPC LAST! I have returned items that have had conflicts or red flags in the terms and conditions - most recently a motherboard from NewEgg. A quick discussion with a supervisor there got me excluded from the restocking fee.
4) Make copies of EVERYTHING, don't send originals unless required
5) Keep a spreadsheet or database of what you have submitted and when, note the offer numbers and deadlines
It seems like a lot of work, but once you get the system going you can get a lot of sh|t you don't need for free

. I still have tons of "filler items" in the attic; these are the FAR things I bought that I added to the order so that I could use a bigger coupon (50 off of 250 instead of 30 of 150 for example).
At one point last year I had over $3k in outstanding rebates and I did eventually get them all. Some required phone calls to the store after the rebate processing company invalidated a valid submission. There are also some companies or some rebate houses I simply will not do theirs. (TigerDirect!!)
To some people its too much of a hassle. I have 500 gigs of disk space in my PC for I paid nothing for because of the time I spent on rebates last year though. I sold a dozen hard drives dirt cheap to friends, and made a couple hundred bucks selling other hard drives on eBay.
Speaking of filler items, does anyone need:
blank CDs
slim jewel cases
corded caller ID telephones
universal remote control
Earthlink Mailstation
Media card reader
56k modem
PC Keyboard
Last year at a "white elephant" style Christmas gift exchange I was able to unload (2) 24x16x12 boxes full of crap like this. These items aren't worth eBaying so they pile up in the attic above the garage.
-Matt-