I used to just buy whatever looked good in the store, and then realize that I had only half the ingredients for any given dish I felt like making, and an alarming amount of the food started going bad pretty quickly... What I started doing was making a list of all the meals I'd plan to make, and then just buying the food I needed for those dishes. Radical concept, but it took me <ahem> okay, more years than I'll admit to figure it out. Now, I usually shop once a week, but sometimes it gets dragged out longer than that, so some of the same tricks for keeping food good longer are useful to me.
I stopped buying more beef than I'd need for one or maybe two meals, and eat those right away. For most meals, I eat vegetarian, but there are a lot of meats that freeze well. We get the pre-frozen boneless skinless chicken breasts (get the Kosher ones if you can, they're juicier). You can defrost these in the microwave, but usually, I'll just rinse off the ice with hot water and then cut the chicken into small chunks while it's still frozen (it takes a good chef's knife, which is a worthwhile investment for many reasons). This works best for dishes in which the chicken is simmered for 10 minutes or more, like in a tomato sauce, but small chunks work well even for stir fries.
Also, I almost never buy fresh tomatoes anymore, except for making salsa or the occasional sandwich. The canned tomatoes are just much better quality than the fresh ones I can get most of the time, and obviously last forever. If you do buy fresh tomatoes, never put them in the fridge: they become mealy and nasty. Store them in a paper bag or other fairly well ventilated container on the counter, in a cool place out of the sun. Try putting a paper towel underneath the tomatoes so that if one starts to go bad, the wet nastiness doesn't seep over into the other ones. Check them frequently and remove any tomatoes that have gone bad and wipe off any scum that got onto the other ones.
Other meats that freeze well: I freeze bacon. You obviously can't separate the bacon into regular slices that way, but I use that sharp chef's knife again to cut crosswise against the slices for slivers or juliennes of bacon - so instead of cooking slices of bacon and then crumbling into pieces to add to a pasta or rice dish, I start out with small pieces.
I've found a couple kinds of frozen heat & serve sausages that taste fine chopped into pieces and added to dishes. Also, Italian sausages (the kind you take out of the casing and crumble) can be used instead of ground beef for certain dishes and I've found it freezes better than ground beef. But you do have to defrost them, as using the microwave ends up cooking parts of the sausage before it's unfrozen enough to remove the casing.
Don't forget canned tuna fish. Not exactly gourmet, but whatdyawant?
Milk should be fine for your two week period - is it really going bad? Eggs, the same. Onions - cool dark place, not the fridge.
Cheese - if you're currently buying pre-shredded cheese, chunks of cheese would last longer, and as somebody else already pointed out, you can always cut off the outside layer of mold on a chunk (but not on preshredded). For hard cheese, just put in plastic bags. For softer cheeses, like cheddar or monterey jack, if you're having a problem with them getting moldy, try wrapping the cheese in wax paper or parchment paper so it's not touching the inside of the plastic bag.
Lettuce - I think here you've got to re-address your priorities. Either you want fresh lettuce enough to shop more often, or you give it up. Others have already commented on this, but to reiterate, the big thing is to keep it from being too moist.
If you ever buy fresh herbs like parsley, rinse them off and shake to remove as much moisture as possible. Then store them in a jar or plastic cup filled with water - like a bouquet of flowers - in the back of your fridge. It will last a lot longer than in a bag in the crisper. Advanced technique, if you can remember: save the plastic bag the parsley came in. After putting the parsley in the fridge, come back some hours later to see whether the residual moisture has finally evaporated, and then put the plastic bag lightly over the top of the "bouquet" like a hair net. This will keep it at the right level of moisture. (If you put the bag on right away it gets too soggy.)
I use lots of frozen vegetables and dry foods, like huge bags of rice and lots of dried pastas and frozen pastas (e.g., tortellini) - just add onions, garlic, and canned tomatoes, mmm.
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