Sandbag training has been around for years and sandbags are a great conditioning tool. You don't need sand to get a good workout though! A simple rucksack can be used to make an effective weighted training bag.
There a simple few rules to follow:
Use What You Have
You don't need a special kind of rucksack to make a good training tool, though it's stands to reason that the sturdier it is the better!
Weight can be added to the bag in the form of small neoprene dumbbells, ankle weights, pebbles....anything. Water bottles can be used too, but just beware that if the workout involves dumping the bag on the floor you might have a mess on your hands!
I often use my 10kg weighted vest in my rucksack for ease of loading and unloading. Another more permanent solution is to buy rubble sacks from a DIY store, fill them with sand, double bag them,then tape them up using duct tape. Making a few separate inner bags also means you have an easily adjustable sandbag.
Fill The Bag
No matter what weight you're going to be making your bag, always make sure you fill the bag.
After adding your weight to to the bag, pad the rest of the bag out with towels or clothes and make sure your weight is padded all around. This will ensure more even distribution of weight and provide protection for you and the weight inside.
Tie Any Handles Up
The final point is to make sure you tie any straps on the outside of the rucksack out of the way - tie them together or use bungee straps. This ensures your safety and minimises fuss when you're working out.
That's it - your good to go!