We usually only buy fresh herbs now, since there's just no way dried basil can compare with the real stuff. But if you're not planning on eating that basil-tomato-mozzarella sandwich for a few more days, here's how to make any herb last a few days longer...
If you're planning on using fresh herbs within 24 hours:
Snip the stem with scissors, so you have a fresh base. Place in a glass or small vase of water, like you would for flowers. Leave them unwashed until it's kitchen prep time.
If you're planning on using them within a week:
Follow the same concept as above, but wrap them loosely in a plastic bag. (A great way to reuse one!) Tie off with a rubber band or twist tie and place in the fridge, changing the water daily. Rinse herbs just before eating.