Look at the junk mail in your mail. Sweepstakes forms, mail-in orders, etc. Try tofake that look. A watermark is always a good touch for these documents. Use the fonts a business would use and word your letters in a similar fashion.
Illusion is everything.
The information on these should include social security numbers. Another good idea is to say that you'll need a password to verify the prize with a voice call. Hopefully it'll be the same as their net account password. It usually is. Yes, people actually fall for this stuff.