Go to pampers.com ( I have added the link at the bottom) and sign up. You will get coupons every 3 weeks, or sometimes more often, to save $1-3 off a jumbo package of diapers.
Also pampers has a gifts to grow program. You earn points by buying pampers diapers. All you have to do is put the 15 digit code in and start saving points. 110 points usually gets you a $10 gift card. They can also be redeemed for a 12 month picture calender and a few other things.
Also you can sign up with huggies.com. Huggies also sends coupons in the mail, just not as often as pampers.
Both sites will also send a newsletter to you with information on where your child should be doing for his age. Which is always fun to read.