Here's an RSVP posted on Reddit this morning, in which Peyton Manning regretfully declined to attend a fan's wedding. Such a polite young man!
(Peyton's apparently one of those who doesn't understand how to use quotation marks. Unless they're scare quotes. "Suuurrre...'Best wishes,' suckers.")
Digging through the old inbox, a pattern emerges. The surest way to get an athlete's autograph appears to be to send them some sort of formal note to which civility demands a reply.
To wit: Tim got married this summer, and invited then-Blackhawk Dave Bolland to his wedding. Bolland promptly got back to him.
Sometimes you get even more than a signature. Tim (a different one) invited Ryan Braun to his wedding. This July, Tim got back an entire package from Braun. It included the RSVP, a handwritten note, an autographed baseball card, and signed batting gloves.
For an even better chance of success, buy an athlete a gift for his wedding. They have to send you a note. Both Jay Cutler and Robert Griffin III sent thank-you cards to fans who bought them items off their wedding registries. While in each case the notes were written by the brides, RGIII autographed his. And Cutler's wife is Kristin Cavallari, so that's not too shabby.
It needn't even be nuptials! Reader Joel dug deep to find a old note from Joe Montana, respectfully declining to attend Joel's bar mitzvah:
There's a natural culmination to all of this, and I think I know what it is. If you can get Dikembe Mutombo to RSVP to a bris, we'll send you some sort of prize. And if you can get Dikembe Mutombo to attend a bris, you won't need our prize because you'll have won at life.