The first thing to mention is that rucksacks aren’t waterproof, so you either need to use some kind of liner, or pack things into waterproof bags before putting them into your rucksack (Some rucksacks now have rain covers and these are useful as they keep the outside of your rucksack reasonably dry, but rain still gets down the back of them, so you should still take the additional measures to help ensure that your kit is dry at the end of your days walking, no matter what the weather has thrown at you)
Personally I use the Rucksack Pro-Tector as apart from it protecting your rucksack on the journey to the starting point of your walk, it has valuable second function as a water resistant liner for the inside of your pack.
The most common myth is that you pack your heaviest items into the bottom of your rucksack
So – Ideally you want have light items in the bottom, the heaviest items in the middle and light items on the top. The reason for this is that the centre of balance of your rucksack will reflect the centre of balance of your body – If heavy items are packed in the bottom, this pulls your rucksack down and increases the weight on your waist strap, therefore you have to keep this overly tight to prevent the rucksack slipping down and when this happens it increases the weight on your shoulder straps causing you sore shoulders as well as an increased risk of back pain.
You also want to ensure that you distribute the weight evenly left and right inside your rucksack, therefore keeping it evenly balanced – You would be surprised at the number of walkers that I have seen wearing rucksacks tilted over to one side, this putting extra strain on one side of the body and again increasing the risks of back and shoulder pain.
Finally, once your rucksack is fully packed, you then need to fine tune the adjustments.
Before putting the rucksack on, ensure that all the compartments are buckled / zipped up and compression straps are adequately tensioned
Then put your rucksack on - The waist strap needs to be snug enough so that it is carrying the bulk of the weight of the rucksack, the shoulder straps need to be reasonably snug, but not over tight so that there is a slight gap when standing between the shoulder strap and the top ofyour shoulders, you should also pull your chest strap reasonably tight to help prevent any movement. The bottom tension adjusters should be pulled in evenly so that your rucksack is a snug fit to the waste strap and the top tension adjuster should be pulled in so that the rucksack is parallel to your body when standing upright.
As you start to walk, you will no doubt find that small adjustments are needed, getting the ride height takes a little bit of doing, there isn’t a simple answer to this but personally, I find that a higher ride height is more comfortable than a lower one
When ascending a big hill, it is worthwhile loosening the top tension adjusters and letting the rucksack fall back a centimetre or two as we naturally tend to lean forwards when going uphill, so by loosening them off a little it keeps your rucksack upright.
I Hope that you find the above info useful - It might well sound like you have an awful lot to do before even taking the first steps of your walk, but I would Definitely Recommend that you choose the right rucksack and pack it carefully as an ill fitting rucksack is something that can Ruin an otherwise wonderful trekking experience