Found this at A Thrifty Mom and thought it would be helpful information.

Buy one get one free sales- When a store has a buy one get one free sale, this is a great time to use your coupons. You can use 2 coupons. One for the item you are buying and one for the item you are getting for free. WHY, you need to remember that a coupon is like cash. The store IS getting paid for that product, by the Manufacturer. So the store gives you the second item for free, but they can turn in the coupon and get paid for that item they just gave you for free. Example- chips are on sale buy one get one free. You pay $4.00 for the first bag of chips, and get the 2nd free. But you have 2 coupons for $1 off a bag of chips. You can use both of those coupons. One coupon is applied to each bag of chips (although one bag is considered free) Both coupons are applied to the total sale price. Bringing the total to $2 for 2 bags of chips. You would have paid $8 for 2 bags of chips, but using the Buy one get one (BOGO) free sale you have saved $4 PLUS $2 in coupons.

Stacking your coupons- Stacking your coupons is when you are able to use 2 coupons for one item. You are able to do this when you use a in-store coupon ( a coupon that the store prints, normally in the weekly ad. It will have the store logo on it) along with a MFR (manufacturer coupon). When you use the in-store coupon and the MFR coupon together this is called “stacking” your coupons. So watch for those in-store coupons, and see if you have a MFR coupon to go along with it.

Forum Co-Leader for Safeway