It's all in the title really! We're about to really tighten our belts, but DH and I both tend to graze the cupboards. I know what the planned meals are and what ingredients we need, but DH had a cheese and cracker binge last night and we barely have enough cheese left for our cauliflower cheese tomorrow. Right now it doesn't matter, but when I don't have enough food budget to buy more I'm going to get grumpy. So I think it's best to work out a strategy when everyone is calm.
(I dread to think what happens when you have teenagers - I can remember my brother (who was a teenage athlete at a national level) eating a whole box of cereal as a casual snack between dinner and his "something to round off the day" at bedtime!)
Does DH generally know what your plans are for meals? If so, he should have the sense not to eat the ingredients, but otherwise you may need to label things so he knows what is available for grazing and what isn't.
We tend to have a plan for the week, and DH knows if I'm planning to use something like cheese - it's one of his favourite snack foods, but he's good at remembering when we need it for something specific!
Can you separate out what you need for meals, so say, grate the cheese, put it in a food bag and write "cauli cheese" on it, or slice it for sandwiches and do the same, then anything left is fair game and when it's gone it's gone. It's a pain in the arse but at least you will have stuff for meals left?