Are you looking for a daypack or a framesack? In either case, I suggest cordura as the fabric of choice, perhaps coated cordura.
http://www.gossamergear.com/cgi-bin/gossamergear/myog.html has a free pattern
http://www.questoutfitters.com/patterns-packs-cart.htm#Pack%20Patterns
http://www.seattlefabrics.com/green.html
http://www.seattlefabrics.com/rainshed.html
http://www.seattlefabrics.com/travel.html#RS150%20Alpine%20Rucksack
There's also: http://www.ericas.com/sewing/patterns/81781b.jpg but I wouldn't trust it with much weight.
The other way to think about the problem is to gather up the stuff you want to carry, and think about how you want it distributed in/on the pack, then make yourself a custom pack pattern. Work it up in $2/yd fllatfold fabric from wallyworld, then when you've got things the way you want them, take it apart and use it to cut a pattern of your "good" fabric.
If you're carrying much weight, don't forget a hip band... it's so much kinder to your back and shoulders.
