This is another of those no-brainers to me, but it's also a green tip. And not everyone is in this mindset, so I thought I'd share.
I am very frugal about the sandwich and snack bags I buy, and mostly use the flip-top variety (usually with twist ties) because they're quite a bit cheaper; you can get a lot more per box than the zip-lock kind. But regardless of what kind of plastic bags they are, I'm constantly reusing them. They generally wash out very well, and dry just fine when you prop them open. Very rarely do I throw one away after it has only been used once.
Bread bags (or gallon size bags, depending on what I have) I reuse and reuse until they get holes in them. And the white grocery bags we get we use as a liner in our small trash can--the other colors we recycle. (Yes, I could eliminate that altogether by using my own reusable bags at the store, but we do need some plastic bags for other purposes.)
I also use lidded containers for almost everything in packed lunches, so that cuts down on the number of bags, too--and my husband always brings his bags home in his lunch bag so we can reuse them.
I've never done the calculations for how much money we save by doing this, but I know it makes a difference, not to mention it puts less of a negative impact on the environment. In this way I only have to buy a new box of bags--of whatever size--a couple times a year.