I don't know if you have any way to do this, but if you do, try different camera backpacks on before you buy one. I did, and found I liked the Tamrac Expedition better for fit. So that is one thing. Another is if you do not have one of those goofy looking photographer's vests.... get one! I have one and wore it when I went on my waterfall trip. The thing was invaluable!
Another thing to think about... weight counts! Every ounce in that pack is one you must carry. If you are the rugged, hiking, outdoors-y type...no problem, but if you are not...find ways to reduce what you are carrying & once you get all the things you are taking, pack it up & carry it around. Make sure straps and hip belt are properly adjusted & that you can carry everything you want to bring! I had to leave my Manfrotto/Bogen home & take a lighter weight tripod to keep from falling over like a turtle! haha
That's it for my advice!