Q. How do you maintain fresh herbs from the supermarket? I use just a small amount of, say, cilantro, and the rest just goes bad. Can I use the rest as dried herb?
Fresh-cut herbs are similar to fresh-cut flowers. Water will keep them fresher. I keep a small vase in the kitchen for bunches of fresh herbs. Trim off the ends of the stems just as you would cut flowers, put them in the vase or a small glass, then put a plastic bag over them and refrigerate them. If you change the water regularly, many herbs will keep great like that.
Fresh basil keeps better in an uncovered jar of water out on the counter. And I learned one exception for the fresh-water rule recently: Fresh dill. It hates water. The stems get slimy and the fronds wilt if you try that.
For herbs such as dill or packages of herbs that aren't cut with the stems on, you can wash them, spin them dry in a salad spinner and then wrap them in a paper towel. Tuck them in a plastic bag (leave it open to let them breath a little) and keep it in the vegetable drawer of the refrigerator.
You can dry most fresh herbs, but some don't dry all that well, particularly cilantro. The easiest way to dry herbs is to spread them on a screen and leave them out in the sun for a few days (bring them in at night). But I've also done it by just spreading herb leaves on a baking sheet and leaving it out on the counter for a day or two.
You also can bundle and hang them; which takes about a month.
But the best way to keep a supply of fresh herbs is to simply grow them yourself. Most herbs do great in small pots in a windowsill.