When is the best time to book my hotel stay for my Las Vegas Trip on April 16-19?

I couple friends and myself are going to Vegas in April and im wondering when is the best time to book. I put myself on the mailing list for most of the major Hotels on the strip. However, it always seems that the best deals that include VIP club pes, buffets and free casino play, usually come along with the stipulation that you are planning on going within the next couple of weeks. Our Airfare has been booked and paid for months back and i'm afraid of waiting too long to book a hotel and getting burned. A coworker of mine who goes often keeps telling me I should wait it out since our economy is in recession and Las Vegas isn't drawing the crowds like it used it too, so the deals will keep getting better? Any body have any advice?