It depends on the situation. I very, very much hate getting ma'amed or being called a lady. Because FTM reasons!
If the situation is fairly casual, I'd stick with folks, guys (depending on region), and you.
For anything formal, I'd ask for a name and use that. I don't ever use sir or ma'am, but I'm not often in situations where that would come up.
The worst is the fumbling and awkwardness. Try to be clear and make eye contact.
If I'm trying to get somebody's attention, I've resorted to "You! With the red shirt!" Rather than try to guess a gender.