What I am about to tell you, I am lead to believe, is the correct way. Now I don't want smart arses ringing me up to tell me that I've got it wrong - it works well for me and just about everyone else I know, so go off and be smug in your own little world!
The most important thing to realise is that the weight is not borne on your back. Your hips and shoulders should be doing the work. If it is a bit of a heavy bugger, and trust me, it will be in the end, never ever ever ever ever ever pick it up with a bent back. Now we've all heard this before and laugh it off, but this Swedish guy travelling with us in the Red Centre of Australia put his back out and had to lie (in agony I should add), on the back seat of the minibus for three days as the rest of us climbed Ayers Rock, walked Kings Canyon, The Olgas etc. Needless to say, he wasn't too happy.
So, how do you do it?
It is simply a matter of trial and error. Don't take shitty advice from mums and dads and others who don't know what they are doing, but most importantly, be honest with yourself. Give it a go and find the comfortable (safe i.e. straight back) way for you. Now you have a full, heavy backpack on your back, this is what it is going to feel like.
The modern day backpacker doesn't actually wear his or her backpack much anyway. As it is all so easy to travel around it is now just a matter of taking it to the bus and putting it in the luggage hold, or transferring it from there to the plane, and from there to another bus which takes you straight to the hostel. Nevertheless, there will be a lot of humping going on when you are away, so you have to make sure you are in a position to manage it. You may also point to the fact that I have already advised you to leave with it half or three quarters full. Fair point. The problem is... you will fill it.
A lot of backpacks come with wheels these days so you don't even have to carry it! Again, it's what feels comfortable for you.

It is vital to fiddle with it and make it as comfortable as possible and then wear it around the house or go for a fairly long walk with it on a good week before you leave.
You will soon realise why it is necessary to pack soft things in behind your back - as having a shoe or sharp corner sticking into your back can be as comfortable as trapping your balls in a car door... and just as annoying... as, in both senses, you have to take everything out and repack it all again!!
You have time now. Tweak and fiddle, fill it and empty it, wear it to bed and sit in it. Is it the best one for you? If not - go and change it for one that you are happy with. Make sure that you don't damage it, keep all the wrapping, take it back to the shop and find one you like. This is the most important item that you're going to take away with you (apart from your lucky pants), so make sure you have a good one that you're happy with.
Mine has now been hundreds of thousands of miles and, although it is a bit of a wreck now, it has done the job well. It will last you for years, so don't worry about the cost so much. Get it right.
I'm going to highlight the main important point again to make sure that the message sinks in with you. The backpack is the most important item you will have to buy and take with you. Don't be fobbed off by your dad's 1962 green issue Scoutmaster Alpha 2 - it'll probably be canvas, heavy, small and with more rivets on it than the Sydney Harbour Bridge. It'll be classed under the title of 'Rucksack' and should stay with the ramblers and bespectacled scout masters of this world ...and off the backpacker trail.
If you've never had or used one before, don't be shy... admit It.
Get help from the professionals and/or the guys in the shops. Let them take you through what your choices are and the differences between them. If you have time between when you buy it and departure, don't be frightened to take it back and exchange it if it turns out to be wrong.
I know I have gone through a process here as to how to get started, what the backpack should look like etc... however, this is guide only please. Get cracking yourself and find your own technique. But please, for me... watch your back!