I have sculpted small clay items from scratch, I can do it if I'm highly motivated. Other times, I'm in a hurry, I need a bunch of pieces exactly the same, or I don't think I can do the subject justice. In those instances, it's good to have a mold to use.
The best molds I've found are from Sweetbrier Studio. These flexible art molds are made of silicone rubber, and are very easy to use. With proper use, molding materials are easily removed from the molds. Polymer clay, air-dry clays, silver metal clay, soap, plaster of paris, hot glue and UTEE may all be used in Sweetbrier Studios molds. (Release clays from molds promptly, do not allow to cure in molds. Release agents may be needed.)
The faces above were all shaped in Sweetbrier Studio molds, using various air-dry clays. Sweetbrier Studios has many other molds to choose from: birds, bears, flowers leaves, Christmas, dolphins, hearts, angels, with new ones created regularly. The standard size molds are 3 1/4" x 4", and the mini molds are3 1/4" x 2 1/4". Sweetbrier Studios website has ordering info., tips, techniques, projects and more.