My method is similar to Eva's. Trim off all usable leaves, and put them in a dishpan
filled with cool water. Swish gently to clean and lift into a colander. Repeat rinsing as needed. Spin dry in a salad spinner. Put single-recipe amounts in individual plastic bags, and gently squeeze out excess air. Put the individual bags in a large freezer bag, and put in the freezer. To use, snip the leaves with scissors while still frozen right into the pot. As long as the cilantro is going to be cooked, this method works fine. If you need fresh, raw cilantro, thawed previously-frozen cilantro is not going to cut it.
I've used this method with basil, parsley, and tarragon too.