Oh yeah, some more tips.
-green leafy veggies (spinach, lettuce, etc). I wash and core these items (or separate into leaves, for big items like swiss chard), wrap them in paper towel, and put them in a large ziploc-style freezer bag. Same with fresh herbs, only a small ziploc bag. The paper towel absorbs moisture and keeps them fresher, longer.
-avocados: place in a paper bag to ripen more quickly, or in the fridge as delicious said to halt ripening. If you use half an avocado, remove the pit from the unusued half. Stretch a square of plastic wrap tightly over the exposed flesh. Replace the pit, and wrap in plastic wrap again. The key is to minimize the flesh's contact with air to keep it from turning brown.
-celery: if celery starts to get limp and wilts, place it upright in a glass with a few inches of water in the bottom. It will crisp right up, usually within an hour.
-onions: if you're the kind of person who might have old pantyhose with runs lying around the house, cut one leg off the hose. Place an onion in the hose, tie a knot above it, place a second onion in the hose, etc. until you have a string of onions. Hang the string or store in a cool, dark place. When you need an onion, simply snip off the lowest one. It keeps the skins from shedding all over your cupboards, and keeps them tidy.
I'll probably think of more later.