Depends on your age and skill level.
If you are younger and moderately good, then dress as out of place as possible. Out of place meaning dress like you dont belong there of like a typical hollywood envisioned poker player (visor, dorky tshirt, and take like you think you know whats going on... but be incorrect. Oh and talk mad smack how you watch a lot of twitch poker).
If.you are middle age or older and good at poker, look as middle aged or older as possible. This way when you 3b light the assume you have the goods every time.
If you're a rec player, regardless of age. Dress comfortably and have fun.