In general, shopping for two isn't much different from shopping for six. That's because you typically have little control over quantities. Oh sure, you can usually buy just two baking potatoes or a single onion, but you usually can't buy an eight ounce can of diced tomatoes, half a pound of lentils, or half a chuck roast. So you're often forced to buy more than you need.
Beans, Pasta, and Dried Foods
A pound of dried beans serves four to six, as does a pound of pasta. I keep unused beans in a pickle jar and I bought some air-tight canisters specifically for storing pasta in. In the case of pasta, I clip the cooking directions from the box or package and drop that in with the noodles to avoid over- or under-cooking.
In fact, I keep a shelf full of glass jars around and store dried mushrooms, dried peppers, barley, rice, and other dried ingredients that I have to buy in quantities larger than I might wish in them.
Don't buy the regular jars of spices and herbs if you can avoid doing so. Most herbs and spices have a shelf life of six to eight months so buy in 1/4 cup quantities whenever possible. Before I started cooking for a living I ordered my herbs from Penzey's in the smallest quantity available. The products are supremely fresh and if they go stale before I can use them I haven't wasted a lot of money. (Did you know that if you bought allspice in August at a grocery store it was probably already a year old?)
Stocks & Broth
I often use chicken and beef stock in sauces and braises and, although I usually make my own, sometimes I fall back on the canned stuff. If I use only half a can I'll pour the rest in a pint zippered freezer bag and freeze it for later use. When I need less than half a can I keep Better Than Bouillon beef and chicken in the fridge. The concentrate is highly salted, but I can allow for that when I have to. And on a related note, check your grocery to see if you can find tomato paste in a tube. The tube allows you to use as little as you need at a time.
Pick a Peck of Pickles
Acid and salt are some of your best friends because both strongly inhibit spoiling. Kept in a refrigerator, olives and pickles will keep for a year provided they're submerged in brine. So will most mustards because they contains a high vinegar component. I probably have a dozen jars of olives and pickles in my refrigerator right now, and half a dozen mustards. (Note that deli olives and pickles, because they're not usually submerged in brine, have a shelf life of weeks, not months.)
Salads can be a big problem for small households. It's difficult to eat an entire head of lettuce before it starts to go bad, much less the three or four different lettuces that a really good salad requires. These days I buy the bagged salads because I can almost finish one before it starts going slimy, but the flavor isn't great. In the past, though, I could shop at groceries that had salad bars and back then I'd make up enough salad for about three days. Salad bars aren't just for a quick lunch.
I mentioned not being able to buy half a roast above, and that problem extends to things like bulk breakfast sausage, packages of chicken breasts, bacon, and Italian sausage. In some upscale markets you can buy individual portions, but that convenience costs a premium. I go ahead and buy the packages and freeze what I don't need.
Shopping for two needn't be difficult, but storing unused products becomes a significant factor when there are just two of you. And if you have any tips of your own, let me know.