When you cut a flower to have for a vase cut the stem just above a leaf of the plant. The action of cutting the stem will encourage new growth to be produced from the leaf buds below, and in time to have more flowers. If the stem is too long of course you can cut it to length afterwards. Herbs will produce more growth easily as you usually only cut a small amount. Mint, basil, and oregano for instance - just cut the tip leaves as much as you need. Parsley leaves should be removed from the base. Chives are usually cut by about half their length or to the ground. depending upon how much you need.