It's a pain in the butt. However, if you have a Dick's Sporting goods near you do this:
Get a Dick's Club Card (or whatever they call it).
Watch real close for their sale ads and coupons in the paper. They often run $10 off of $50 or $20 off of $100 deals.
They also often run buy one get one 1/2 off ammo deals.
Buy an Entertainment Book if they have them in your area and Dick's is in it, they have great coupons that's good for ammo.
Use the Remington and Winchester rebates that come out each year for rifle ammo (also gets you more points towards the next gift card).
As you spend money at Dick's you'll get gift cards in the mail (more ammo).
It's a pain in the butt, but the other day I found a receipt that I pain $4.34 for Rem 9mm ammo per box last year!
If that's too much for you then it's Wally World or Ammoman. Both are great options if you want to keep it simple.