Man....
I remember the first time I ever sent away for autographs. I was about 13 years old and I was REALLY into the Detroit Red Wings.
This was probably around 1995 and I took some of the Red Wings cards that I had (Lidstrom, Fedorov, Lapointe, Konstantinov, Yzerman...) and I requested autographs.
I don't remember how long it took, but I received an envelope with a Red Wings Logo/Return Address. I was really excited, but when I opened the envelope there was a really nice letter typed on some expensive looking cotton stock paper and it said something like "Dear Fan, thank you very much for your interest in the Detroit Red Wings. Due to the amount of fan mail received we suggest that you send a particular item directly to the player in care of the team and maybe you will get a response" (it wasn't exactly like that but pretty much).
They also sent a sticker and my unsigned cards. I was such a big Red Wings fan that my WHOLE wall was like a big mural. Red Wings clippings from newspapers and magazines, cards, posters, pucks, jerseys, pictures....
Good luck on the Clemens, but since it was so long ago I don't know that you will receive anything. Maybe if you sent one to him now you might get a response?
Thanks
Mike