Off-road: small pack, no more than 3 or 4 lbs, water, food, etc. On-road: prefer not to wear it, but have a small pack to take on excursions into town, shopping, etc. Sometimes it's better to use a Timbuk2 bag, especially if carrying a laptop.

Oh, and sometimes a hydration waistpack. http://www.rei.com/category/4500633