Smiles... I've practiced them a lot in front of the mirror. The problem with most people is that they only smile with their mouth, and not with their eyes. What I noticed from comparing fake smile with genuine smile photographies is that in the genuine ones, the ones at their picture raise their cheekbones and their lower eyelid a bit. You can see the difference in front of a mirror too.
You say it doesn't feel right... well, if that smile doesn't come from your heart, that's bound to happen. If your neighbor ran over your cat and you smiled, I'd bet that smile wouldn't feel right either, that's part of why practicing in front of the mirror works, you teach yourself to smile even when you don't feel like it.