When possible, continue shopping for deals even after you book your trip. I understand that packaged sun vacations are typically non cancellable but in most other situations, you can cancel your reservation up to 48 hours before.

This means that you should keep shopping for better deals until the very last minute. Hotels, car rental agencies, tourist attractions will sometimes run last minute deals to fill up their spot at greatly reduced rates and you shouldn't miss these. 

Sometimes hotels will offer upgraded accommodations for the same (or very close) price. I once managed to get the presidential suite at the Sheraton downtown toronto for $250 a night. The same type of opportunity can be had for car rentals (in less busy locations).

So never shop shopping.