The nubmer one rule of wise shopping is Don't Buy What You Don't Use (or would like to use but haven't quite figured how, or don't have time to process, etc). Even if it's a good deal. Even if it's a really, really great deal. Actually, if something isn't going to be used, I wouldn't take it even for free. Why waste valuable space? I like it when my fridge, freezer and pantry are well-organized, with space to spare - not so clogged up I can hardly see what's in there.
This rule is especially true for fresh produce (though for us Jews, Passover puts some extra limit on our magpie-like tendencies as well. There is no point to stock up on pasta if Pesach is just around the corner). Just today, I opened the fridge to see ten sad-looking eggplants on the shelf, and one that rather resembled a Petri dish (I chucked it straight into the garbage bin, though perhaps I should have taken a photo). Of course, I can think straight off the bat of a dozen things to do with eggplant. It's so incredibly versatile, it can be made into all kinds of dips, quiches, stir-frys, lasagna, etc... but I already started my cooking plan for the day, and I knew I will not be able to squeeze eggplant into it. Also, for lasagna I would need hard cheese and other things I currently don't have on hand.
So, how did it happen that I have ten eggplants in the fridge, and they aren't exactly new residents, either? Easy; generally, I use 2-3 eggplants per week. My husband, however, finds it hard to resist these big glossy vegetables and buys, on average, 4-5 per week, even when they are not on the shopping list. So... they hang around until they aren't so glossy anymore.
Lesson learned: when buying fresh produce, make a realistic estimate of how much of it you can use or preserve in a reasonable amount of time. For example, you can do a lot of wonderful things with bananas (banana bread, cake, smoothies, etc), but if you aren't into baking right now, or just think, upon honest reflection, that you will not be able to do it, better not buy it. Of course, if somehow these sneaky fruit do end up in your home ("they just fell into my shopping cart and I didn't even notice!"), and your kitchen looks like a fruit market, you can be a good sport and do your best to use it up.
Other useful rules:
Don't buy it if it's not good for your health, even if it's cheap.
There is hardly any point to buy more of something if you think it will only get used up faster. We have this scientific law in our home that states The Milk Will Run Out In Two Days. It hardly matters how much we buy. It will still all be gone, and I'll say, "I wish we could have milking goats again. No, no, of course I'm not serious. No, I don't want to go and have a look at that very cute Alpine doe, because I know I will once again find myself with more animals than I can handle."