$400/month on groceries is actually less than I spend, but I do have a couple of tips for finding cheap groceries in Watertown.
Shop at Aldi on Arsenal Street for staples like milk, bread, eggs, juice, etc. They also have the cheapest canned goods in town. Their dairy products are also very high quality. I never spend over $40 in there, and that's with a cartload of groceries. You have to bring your own shopping bags (or you can buy them if you forget) and "rent" a shopping cart. The place isn't aesthetically pleasing in the least, but it has great prices on food. Beggars can't be choosers. Produce at Aldi is also relatively cheap but they don't have much of a selection and I've never been impressed with quality.
I also do a lot of bulk shopping at Sam's Club, especially on frozen vegetables, frozen meats (like fish and chicken fingers), salad/produce and "snacky" items like huge boxes of Cheez-Its and the economy pack of granola bars for my husband's lunches.
Finally, I shop at Hannaford for either name brand items (I only like Campbell's condensed soups and a certain brand of hummus, for example) or specific produce I need in recipes but can't find at Aldi and don't want five lbs of at Sam's.
I check the weekly ads for Price Chopper and Hannaford online for sales on meat (especially chicken), which I then buy in bulk and freeze.
www.hannaford.com
www.pricechopper.com
New sales start every Sunday
I did try shopping at Super Wal-Mart when I had non-grocery items I had to get, but it is quite a bit more expensive than shopping around at the stores I listed.
I don't think I waste gas going from store to store since they're all within a few miles of each other.
Every Wednesday from May to September (I think), there's an amazing Farmer's Market on Washington Street where you can get beautiful seasonal produce. It's also a lot of fun! Hope this helps.