1. Buying one do-it-all pack? Size up. Bring your gear to the store and look for a model that will handle your biggest trip, then make sure it has an effective compression system so you can shrink it down for overnights and big dayhikes, too.
2. Get the right torso length. Tall people can have short torsos (and vice versa), so measure your back (see how at backpacker.com/torso) and get a pack that fits.
3. Fine-tune load control. Learn to adjust hip, shoulder, and load-lifter straps to shift the weight up and down. Over the course of a long day, hips and shoulders will need some relief.
4. Line your pack with a trash compactor bag for cheap, effective waterproofing.
5. Pack your load symmetrically and with heavier items closer to your back for the best balance.