First off, don’t try to approach her during busy hours, it’s not gonna work. Wait until the bar is empty and there’s no one around. Also, don’t even think about hitting on her when her boss is there. Social pressure is a big deal when it comes to talking to women, that’s why I always say isolation is key. So, make sure you catch her alone.
A barmaid is very busy at work. You can’t take up her time. Have a drink where she works and observe her. Wait for the moment to talk to her. In other words, wait for her to be on her own. Compliment her, give your business card to her and leave the premises. Doing so will maximize your chance of being successful without exposing yourself.