girliefriend, if you can be arsed doing it, one good way to make sure you stay on budget is to go online and plan all your shopping list out first. That way you can see exactly how much everything will cost. If you get to the shop and certain things aren't available, at least you can see how much it might affect your budget by.
(It is a bit time consuming, but it works if, like me, you'd rather go to the shop yourself than buy food online. I don't know why but I much prefer getting everything myself, plus I don't have to pay for delivery!)
I've just gone through YNAB to plot out our monthly budget... I think it's all accounted for! We are a little into the red but as DH gets paid mid-month it should balance out and give us quite a bit left over to go into savings. Provided, of course, we actually manage to stick to it.
Next step is to crack on with a meal plan for the next few days. We've plenty of food in as DH bought a load of fruit and veg in post-Christmas desperation so we should be ok for at least 5 days I think (fingers crossed!).