Answered[ Edited ]
Hands down, prescription strength retinoids (Retin-A, etc.; the downside is that they can be irritating until you build up a tolerance). Retinol (found OTC in skincare products) can be effective, but be sure to be aware of the concentration -- while it's marketed as an anti-aging ingredient, the concentration of retinol in a product is often not high enough to render it effective or to yield results.
Also: Stay out of the sun! Nothing ages faster than the sun does -- no tan is a safe tan. Protect your skin on a daily basis by using sunscreen on your face, and when you're out in the sun, wear a hat and be sure to use an adequate amount of sunscreen (a shotglass-sized amount is recommended for an adult).