One thing you can do if she is mouthy is just rub vigorously on her muzzle....not enough to hurt, but just enough to be annoying, until she takes her mouth away. Do this every time she bothers you with her mouth. It doesn't make them headshy, but does work. I would only ever smack if she actually bites you with teeth (as opposed to baby mouthing).
I also agree that establishing personal space early is a great idea. However, be careful about just pushing her head away....this may turn into a head swinging or pushing game for her. She may be a tad bit young now, but when she's ready you can train her to back up from pressure on her chest and/or nose and use that.