What is the best part of a rosemary plant to trim?

I have two small (maybe 12-14 inch tall) rosemary plants. I love to use fresh rosemary in cooking. Should I take branches from the top of the plant or the sides? I want to keep it alive for as long as possible. I've had other rosemary plants that never seemed to grow back after I took cuttings from them to use. Any advice on how to not kill them would be appreciated.