Here is a good video about how to fit a rucksack. The advice about where your hip belt should be is more in line with a woman’s pack (hip belt should be somewhere around the middle of the top of the iliac crest aka top of the pelvic bone), men will want it a little lower with the top of the hip belt in line with top of the iliac crest. This, however will be a personal preference thing and packs do vary as to what is most comfortable. Of course, in this video the pack is fitted to a woman but the hip belt and height of the chest strap (most women wear it higher for obvious reasons) are the only variations.
The biggest issue I have found with rucksacks (especially borrowed ones) is the height of the back. Get someone to help you (or use a mirror) and look out for the tell tale gap at the top of the shoulder strap – it should be laying flat not pulled up. This problem can also be caused by having the load adjuster strap (the small one attached to the top of the shoulder strap) too tight. Adjustments to the back are often made with a great big bit of hook and loop material and a sleeve down the back of the pack (on the outside) or with straps or sometimes a combination of both. Remember that to fit a rucksack correctly in needs some weight in it to pull it down.
This second video includes how to lift a pack as well as minor fitting adjustments – I found it a little misleading about his comments on leaning forwards because it is inevitable that with a big pack and lots of weight you will lean a little – it is just a centre of gravity thing. He means that you shouldn’t be leaning a long way forwards and if you need to then your pack is not adjusted correctly – he is right but I thought it was a little misleading.