What has worked for me:

- Make your reservation using the Marriott elite phone line. Somehow, when they request an upgrade is seems to work better than using the comments box when you make your reservation online.

- Call the hotel mid-morning the day you are to arrive (after most have checked out but before the afternoon check-ins have started). I usually make up some lame question and then ask "by the way, have I gotten a platinum upgrade..." Sometimes, they will go above the normal Concierge Level upgrade room. YMMV.