What are you carrying around with you every day?
Take your backpack, laptop bag and/or purse, wallet and beltware and lay it all out on the table.
Do you really need all this shit with you all the time? Do some weeding to optimize your kit.
Are you using the right thing to carry it all around? Last time I did this exercise I changed to a smaller laptop bag which sets an upper ceiling on the amount of stuff which can accrete into my routine load.
Don't forget to create the mini-kit that has just the essentials for a walk or evening out. I have a simple little wallet sized pouch that holds my transit passes, ATM card, credit card, paper money, health care card, driver's license, small pieces of yellow paper and space pen. With that, my keys and my mobile phone (with its built-in camera), I'm pretty much ready for anything that doesn't require my computer.
Get in the habit of quickly returning your bag and kit to its base condition – toss that extra change into a jar on your dresser, throw out the wrappers, put the receipts in with your bills, etc. – at least once a week. Your back will thank you.