I disagree about priority mail, unless you can send it certified with return receipt.
The best way is to overnight it or send it by FexEx's cheapest way, which will be able to track your envelope and track who signed for it. This option will tend to be expensive, though. FedEx has a new ground shipping option that is worth checking out.
The way I plan on sending it is to send it regular mail, but get it certified with return receipt. Basically, it goes through regular mail, but you attach a post office post card which the recipient signs and dates. The post card is then removed from the package/envelope and mailed back to you. If you can do this Priority Mail, which is supposed to take 2-3 days, all the better. The cost of this will be around 3-5 dollars.