For guys, stay away from your "Casino Royale" style tuxes. While they may be what high rollers wear on TV, the reality is that many of your rollers look more like Jason Statham. The bad boy look is very in and is something you might want to try.
Next, let's talk about footwear. Stylish dress shoes and high heels may look sexy but you need to remember that you are going to be on your feet a long time at the casino and you need to wear appropriate footwear. Guys can stick with a nice pair of loafers. These look stylish but are much more friendly on your feet. Women can stick with a pair of platform wedges for much the same reason. You get the effect and glamour of high heels without pain.
Finally, consider using your wardrobe to impact how people see you at the table. This is something that applies more to poker players than anything else. How you are dressed will impact how others perceive your style of play and you can use your table image as an advantage during the game. For example, you may want to dress very conservative to give off an image of being tight. This will allow you bluff more often. Keep in mind that this works very well against less experienced players.
The above tips should get you started in picking out a wardrobe for a fun night at the casino. Keep it simple and fun and go win yourself some money.