Personally, we book when we have funds available... So I booked our June of 2012 vacation back in August....

But keep in mind to keep checking prices. My wife and I book through Costco and we booked Maui for June of 2012. I checked the package last week and it had dropped $1,200 in price and they included access to the spa for two every day (this is at the Hyatt in Maui).

I called them up and rebooked the package at the cheaper price and to include the spa access. They did charge a $75 rebooking fee, but well worth the savings and included spa package.

So for me, I book when as early as I can, but make sure that if prices change, you can rebook without penalty. That $75 dollar rebooking fee gave us enough money back to cover the two weeks of eating out and more.