Best travel backpack ever!?

I am going on a three month trip to Nicaragua and have been encouraged to pack about half of what I think I need. So... I'm going to need a large (as large as possible) travel backpack. Any ideas, suggestions, pertinent observations? BIG backpack!

One suggestion.. Don't pack a LARGE backpack. If you're carrying the thing on your back you will hate every extra t-shirt you packed in it.