As for cooking for a single person, I know what you mean. Individual pieces of meat are more expensive (Or unavailable), vegetables, creams, and other fresh produce goes off before you can use it, you put alot of effort into cooking what only lasts for a few minutes in some cases...
So I gave up. I now cook for 2. Or 4. The leftovers go into the fridge for two days and into the freezer at 4. Sure, I'm out of space in the freezer, but eating one of leftovers for lunch the next day, I'm left with two ready made meals (Or sometimes, none at all), and so i'm all set.
As for food buying / saving, I freeze everything. If a pumpkin wasn't used and is about to go off, I chop it up and freeze it. Yes, it goes a funny colour. Yes, it goes squishy. Yes, it's just fine if you use it in a soup, or a puree, or risotto. Carrots can still be made into a Mire Poix, cabbage is a little limp but is OK if you're stuffing something with it (Or you like it soggy with butter like my mother).
I buy BBQ chickens when they're discounted, chill them, separate into quarters and freeze. Once I've used the flesh from one I pop the bones into a ziplock bag and freeze again, ready to make (Not as good, but still acceptable) stock.