Don't throw out the box: Most companies will require you to send in a UPC barcode or a serial number barcode from the product packaging, and will not process your submission without the original barcodes from the box.
Make copies of all your paperwork: This includes your receipt, rebate form, barcodes, and anything else you send in order to receive your rebate. If for some reason your paperwork gets lost in the mail, you'll need this information to have your rebate reprocessed. It's also a good idea to have this information in front of you if you need to call the customer care phone number listed on your rebate form.
Check your postmark date! Most rebates require you to postmark your submission within 30 days of the date of purchase. If you miss this deadline, you will not be able to receive your rebate.