Just my personal opinion. But I've noticed that practicing facial expressions hasn't really worked for me. When I'm feeling confident, charming, charismatic etc., I naturally have a very sexy smile. Whereas, if I'm feeling weak, or "out of state" so to speak, I can't fake a sexy smile for my life. There are a lot of subtle details which are just ridiculously hard to emulate. Ergo, producing the exterior result (the sexy smile) without the interior emotions (confidence) is almost impossible.