Here's a couple of my favorites:
1. Chop up a big pile of onions or red/green bell peppers all at once. Freeze individual portions in ice cube trays. When the cubes are fully frozen, remove from trays & place in Zip-Loc freezer bags. Remove a cube or 2, as needed. This means that I always have onions & peppers ready when I need them & I'm not throwing onions & peppers away because they've gone bad. I also seem to use less onions than I would if I was chopping them fresh.
2. Thoroughly clean a "Sports Bottle" (Dasani/Poland Springs, etc.) after you drink the water. (Sports bottles are the ones with pull-up or twisting openings.) Fill with oil to use for cooking. I have one for olive oil & one for vegetable oil. I buy large containers of oil at BJ's & keep refilling the bottles. When I need some oil in the pan for sauteing something, it's so easy to just squirt as much as I want into the pan.
I'm all about the food!