Though it is quite difficult to get dimples naturally, but there are some ways which can help you, though there is no guarantee that they work...
1. Smile widely which will make the muscles around your 'smile' to get cramps and form temporary dimples
2. Piercing can help too.
3. Pucker your lips and suck your cheeks in as if you have eaten a lemon or something very sour. Do this for at least 20 minutes. (it is NOT scientifically proven that it helps)