For my Alaska trips, I plan them 1-2 years in advance. It is much cheaper that way. Car rentals booked 10-11 months in advance for Anchorage can be $95/week but booked 5-6 months in advance can be $250/week or more.

Booking cabins in highly-prized locations requires advance planning, as does campgrounds in places like Katmai National Park. Many cabins are booked exactly 6 months to the day in advance of arrival.

I am already sketching out an Alaska itinerary for 2011 so that I will have it pretty firm 11 months in advance, when I can seek frequent flyer seats then start looking at rental cars, etc.

As was mentioned above, the planning is almost half the fun.

For many other trips, I don't plan them nearly as far in advance.