Anyway, it's basically mailing something (a card, picture, magazine cover, whatever...) to an athlete with an SASE (self-addressed stamped envelope) along with a well written, thoughtful letter asking for an autograph. It's simple and super-cheap. This has always been an important factor because I have never had a bunch of money to spend.
I have gotten far too many autographs to count through the mail. However, some very big names will surprise you and be receptive to the act. Within the past couple of years I have received: Yogi Berra, Josh Hamilton, Cam Neely, Sparky Anderson, and CC Sabathia. All for the price of a stamp...which, if you are good at math, is a pretty friggin good deal.
The key is writing a good-ass letter. Be original and thoughtful, don't curse, and don't be afraid to kiss a little ass. A little flattery goes a very long way.
TTM is a great hobby for kids to get into. If you are a parent, take note. This hobby is cheap AND it gets your kids to actually write a letter. You know, the act of physically taking a pen, pencil, or crayon and writing on paper...not typing (typing a TTM letter is LAME!). Kids will actually be begging to be allowed to go get the mail.
Well, that's just a quick two cents on a cheap, fun part of the hobby. Good luck to you all.