www.coupons.com
www.redplum.com
www.smartshopper.com
My husband also brings home the coupons from the Sunday paper where he works since it goes in the recycle bin there. If you are brand-loyal to anything, check their website and sign up for emails, promotions, coupons, specials, etc. If you shop a few specific grocers, see if their websites list the store prices and make sure you get their weekly flyers or look at them online. I have tried and WILL NOT shop at Aldis (I know a lot of people who do, and I've tried, and there is no point for me), but I shop at Wegmans, Sam's Club, and now Tops since there is one right next to my work. I check price per pound or ounce to see which brand/store is the best deal. I do not make more than one major trip to one main grocer and we plan our meals accordingly.
I have for the past 2 months been keeping itemized (by department, item, quantity, and cost) lists of our purchases to see what we purchase, how often, and how much it costs to see where the leaks are or where we are better off buying in bulk. That has been pretty helpful for me. I also have not bought cleaners except for dish soap in at least a year since I am makign mine with vinegar, baking soda, and borax mostly, and make my own laundry soap, which I like better and find to be dirt cheap. =)