In reading others experiences the upgrade success rate varies quite a bit from property to property. I think a lot of it has to do with volume. Volume meaning length of stay and how many people from your company are staying at that same hotel. The type of work I do usually requires at least 6 months in the same city with normally 5 to 20 other consultants from my company. So I guess you could say we have a little "pull" at the hotel we choose to stay at

As a result I almost always get on the conceriege floor. Getting upgraded to a suite is pretty rare though in my experience.

Some successes:

1) Plaza Marriott, Kansas City, MO. - Upgraded to Presidential Suite...twice
2) Downtown L.A. Marriott - Spent three weeks there...Junior Suite every night
3) Oklahoma City Marriott - Worked there with a group of people for two years. Not only did I get on the Concierge level every single week, they kept the same room for me (at my request). ^

You can't win em all:

1) Oakland Downtown Marriott - Spent three months there. Most of the time I couldn't even get a room in the hotel and when I did I never got on the C level - not once in 3 months.

I think it helps if they know you at the hotel. Once you've been there for a few weeks the hotel employees will recognize your face. I always make it a point to introduce myself to the hotel manager when the opportunity presents itself. And don't forget the front desk staff. They are the gatekeepers so be nice to them