You have to know what angles are most flattering for your face. Try looking in a mirror and tilting your head slightly in different directions until you think you've found your best looking angle and then when you take a picture make sure you angle your face the same way at the lens
Being photogenic is being able to know all your good angles, good poses and perhaps knowing your lighting well. People who are photogenic basically just are posing in a natural looking way. If you look at all the photos of photgenics you will see they have only a few looks and you will notice repeating paterns of the same things. Either that or good photoshop skills lol
Well good posture and a smile helps, so you can always practice those things in the mirror.