I usually make my reservations on the Marriott website, and in the "comments" box will put "Request Platinum {or Gold, depending on my status} upgrade, if available, please."

I don't know whether it makes a difference, but I do get upgraded a lot.

One other thing I do is, when staying at a hotel where I'm not familiar with the numbering system, or when I am familiar with the system and recognize that the room they have marked for me is not an upgrade, is to inquire, "Do you know whether my Gold/Platinum upgrade came through?" Sometimes they'll say, "I'm sorry, but the hotel is pretty full tonight" but other times thay'll say, "Wait a minute, let me check" and come back with a new room number.