Here's my bit:

You don't need it! It's amazing how little we really need to get by and live a happy life. The human race survived millions of years without the iPhone.

To that end, here's some tips:

1.) Don't tempt yourself. Don't hang out at the mall for fun. Don't read catalogs. Don't spend hours at Amazon.com. Giving yourself less opportunities to see things to buy gives you more time to enjoy what you have. Take a walk, watch a movie, or call a friend instead.

2.) Do the "cart purge". Anytime you're at the store, stop right before you check out and remove at least one thing from your cart. You didn't need it.

3.) As another poster mentioned, follow the 24 hour rule. If you're making a large or unplanned purchase, tell yourself that you can come back an get in 24 hours. Half the time you will have forgotten about it by the time you reach the parking lot.