Here's a tip I recently put into action. If you want to train with some actual pack weights with your pack (40-80lbs) here's a good cheap way to get that together.

Get a bathroom scale. Go to Lowes/ Home Depot and get sandbags like they use for flooding get 3 or 4 of them. Mine were .62c each. Get 2 50 lb bags of play sand or gravel. Should run about $3.50 a bag.

Use your scale and fill the bags up to 20 lbs each or so. You now have a 80 lb weight set of 4 bags of 20 lbs each for under $10.

You can load up your pack with weights that are more malleable than a dumbell or plate that wont' rip your pack up or poke you in the back.

Its also good to vary your training so you don't use a heavy pack every time and hurt yourself. I'm doing stairs for an hour with my frame pack on, 3 times a week, 20,40, 60 lbs to get in shape. The sandbags are easy to put in and take out and change the weight quickly.

Just be prepared to get some strange looks if some asks you what you got in there and you say "60 lbs of rocks."