Save the receipt and file it with your warranty in case you later need to prove when you bought the item or that you're the original purchaser.
Don't bother sending in the warranty postcard -- unless you want to. The manufacturer's just fishing for names for its mailing list and you're covered whether you send it in or not.
Perform any maintenance or inspections required by the warranty so you don't void it.
Before purchasing an extended warranty, find out the name of the company offering it and do a background check with the Better Business Bureau.
Your county or state consumer protection office may be able to help. In extreme cases, you could file a lawsuit citing the Magnuson-Moss act, which is the law that governs warranties.