One of the easiest ways to make a new plant is layering. Layering is where you allow the plant to create new roots on a branch while still connected to the mother plant. The advantage to layering is the connection to the mother plant. It continues to feed the offshoot branch allowing it to form the new roots to sustain itself. Many plants do this naturally and you don't have to do anything special to create the offshoot. Rosemary does this really well.If you want to help it along make a small cut into the branch being careful not to sever the branch then put a toothpick in the wound to keep it open. Finally use landscape pins to pin down the branch to the ground. With rosemary you could get away with only doing the last step. In a few months you will have new plants to pot up or move to another area of the garden. Just cut the stem that holds the new plant to the mother plant when you are ready to plant the offshoot. It may help to cover the rooting section of the plant with mulch or a small mound of dirt.
Rosemary works great in herb gardens and trimmed up in topiary form, not to mention that it is perfect with onions and garlic in potato dishes!