I've noticed Science Diet coupons can be hard to come by, so when I found this I thought I'd share. You just have to sign up to be a member of their club (not much spam, I promise) or sign in if you are already signed up.
Science Diet $5. coupon
Another way I have found to get Science Diet coupons is to call and ask. This will only work a couple times a year. They will say they don't give out coupons, but will ask for your address and usually send you a couple anyway.