Simplest:
-Even if you don't cook completely from scratch, one thing I've found - learn to make your own baked goods. All of them. I can't believe the cost of bagels (and how many I bought!) when I know full well how little it costs me to make them at home (and they're BETTER!). If you normally only buy a decent loaf of bread each week and a package of tortillas/pitas/etc., by making them at home, you'll save about $5 bucks a week. If you buy fancy-shmancy bagels/muffins/etc., you'll save much more doing it on your own. Sounds small, but it adds up. And making baked goods is NOT as time consuming as it sounds...you can mix the dough/batter up the night before, in most cases, and let it rise in the fridge overnight. Plus, it's much more appealing to have a homebaked banana muffin (made with that leftover brown banana, and 1/2 cup of yogurt that was sitting around

) than another piece of boring store-bought toast.
-Condense ALL your errands into one run a week, if possible (I have to do two - I understand how it is with toddlers and their ability to stay out running errands). Or at least to/from your way to work. You'll be surprised how much you save on gas (I just started doing this in the past couple of months, and it's really made a difference). Plus, it forces you to be more organized re: what you're going to buy, and why (ie...you plan your meals/snacks out so that you *won't* have to run out again - and then you resist temptation -the less time in the stores, the better).
-I don't know your living situation, but, is it possible to take in a boarder? Maybe a grad student or something along those lines? You can check craigslist...Like I said, I don't know your housing situation - but, could you put the kids all in one room to free up an extra bedroom, or something like that? You could easily get anywhere from 2-400/month doing that...It doesn't have to be forever...through the end of the year, or something like that?