I flew SW two years ago and in a five month period of time earned three free tickets as a RR member. I haven't flown them since and last week I received a letter describing this same offer.
In my case (and like SGGOLF), the letter stated that SW "missed my business" and wanted to offer their thanks for flying SW again in the near future.
I read that my participation is automatic and there is no sign up process. There is no reference number on the letter nor is there any telephone number to call.
Dilbert: My form letter reads that I need to fly two RT to earn my free ticket and then those two RT count towards the next 8 RT award (so I have six left to go for that reward). I do not see where you're allowed to participate in this bonus more than once.