The correct answer is, of course, to bin it as you are no longer 12 years old and at school, and to buy something with a single strap that goes over the shoulder.
Notable exceptions to this rule include any sac designed to carry fluids for those marathons, or dressing up as a Ghostbuster with proton pack for parties.
Tell you what......you show me (with your back) you carrying round 30kgs of worldly possessions round the highways and byways of Oz, whilst desperately running away from rabid koala's for 4 months ..........on one shoulder .........and I'll admit that I'm a big bufty for getting a proper one!