I have seen recent posts indicating that some folks are having trouble booking rooms using points in high season. Here are some lessons learned from years of trying:
Call the hotel or resort directly first. A reservationist at larger hotels can normally tell you what your chances are of getting in using points, or if not what needs to happen (cancellations, previously booked conferences, etc.) for you to be put on a wait list.
If you have a paying history at a particular property, or you've hosted meetings there, mention that.
Check back often, as many times as you can. I was trying for a MR stay at the Marriott Marquis in NYC in prime time and one night, after about thirty failures, I got the points-stay for the time that I needed. Like the airlines, Marriott opens and closes discounted and other availability.
Check this year especially since major conferences are being scaled back or canceled: CISCO canceled its summer 09 conference in SF for example, freeing thousands of rooms there, many at Marriott hotels. This is the year when many hotels will be making deals.
Be nice, that goes without saying. As a Platinum Premier I never have to ask for an upgrade since the offer is normally made as soon as I walk in, even for the first time, when they pull up the reservation. I've seen other folks (status unknown) get agitated and demand things - not a good idea and bad for your blood pressure as well.
These tips may help, and if they don't well...you tried and you'll try again.