A working-poor friend also used to buy steel-cut oats, to eat breakfast for pennies.
If you buy fresh vegetables, learn how to keep them from spoiling on you.
With canned soup, instant ramen, and a lot of other packaged foods, pay attention to the sodium. (I avoid a lot of those, or end up diluting it, or, say, using only half the seasoning packet.)
Where you shop matters to budget. Where I live, WFM has bought out, or driven out, most of the other grocery stores (and one of the remaining stores prices similarly, but without the cachet), but a Market Basket is just a short hike over the train tracks.
For specialty groceries, a small grocery store serving some ethnic group for which that's a staple, can be cheaper than most broader grocery stores.
Don't get scurvy, or other nutritional deficiency. If you can't make sure all your nutrients are covered, I've guessed (I'm not a nutritionist) that a mainstream multivitamin pill can't hurt.
You should also count your calories, to make sure you're getting enough. Once you start being more penny-pinching with eating, you might find yourself getting hungry less, rather than more, and it's not hard to slip into caloric deficit that will make you sick. There are calculators on the Web for how much you should intake.