the first two i'd look at (b/c they'd be totally under your immediate control) would be groceries and spending money (especially since each individually are the exact amount you're coming up short on at the end of the month, right?). obviously, you can't cut them both to the bone, but you could whittle away at each one. consider all of your purchases in your spending money category. i too am a sahm living with my dh, 2.5 y.o ds, and have one on the way. we have $150 in our "entertainment" budget each month with $50 of that going straight to DH as he is a bigger spender than me. i will talk myself out of a pack of gum! i don't feel deprived - i just have different priorities and i enjoy seeing how far i can make $100 go for our family's entertainment in a month. for us, entertainment means date night once per week for me and dh (this may mean a restaurant dinner, takeout, or just eating a homemade pizza at home), entrance fees should we go someplace that requires one - basically any money we spend "out and about" that's not groceries or gas. clothing does not come out of this, nor do large toy purchases, but small ones do. i.e. son gets a matchbox car, it's gotta come out of entertainment. budgeting for a treehouse is a different story b/c we've managed to create large savings every month for that type of thing.
i just recommend reconsidering all of your purchases. i don't mean to say deprive yourself, but really think about whether or not you need a new top today or a new pair of shoes, for example.
groceries, i'll have a harder time with as I am NOT into trying new recipes and doing anything gourmet! good for you for being adventurous with food (i've never been that way and will cross off a recipe if it has one ingredient i don't like!

), but i think for me, meal planning is the ticket. deciding what we're going to have for the week or month and trying my best to stick to that has helped. i feed myself (pregnant and lactating), dh, ds, a large dog, and several cats PLUS run the household (greeting cards, lightbulbs, detergent, soap, etc...) on $350 per month, sometimes $400. i don't go off my meal plan much if at all.
good luck!