I like sand...it molds to the dogs body rather than weighs or water bottles, etc. if you are just walking, I'd probably not do more than 2lbs in a backpack. Don't want to stress the back. I like to put the sand in for only the first mile maybe and that is just to get the dog tired and working, then I take it off.
If you are talking about dragging or pulling weight well then start off just dragging a chain and work your way up to weights. You can use a drag sled or use a window weight type thing.
You will find that everyone does things different...depends on what your end goal is.
"It's a nokalokapoopalotadoocanidragaleashalupus. It's a breed of mixed, strained money extorted
practically useless spin off of a once noble breed." --William Williamson