Positioning at the bar (and anytime you’re both naturally facing something and your shoulders are pointed the same way), you should be starting a conversation by simply turning your head to the right and starting to talk.  You can even put your shoulder a bit in front of her shoulder but never behind her shoulder unless you were already standing behind her when you started.

The big mistakes guys make when standing at a bar are:

1.  Turning to a 90 degree angle facing the girl (showing maximum possible interest and no interest in the bartender/thing going on in front of you)

1.  Putting your back up to the bar, leaning back on it, and then starting conversation like that.

Instead of these, just walk up to the bar next to her and turn your head towards her.  Simple.  Then be conscious of her shoulder position when it changes.  Also remember you’re at the bar, so be conscious of what the bartender is doing and how far she is into her drink ordering process, because this will let you know how much time you’ve got before you could face an obstacle in your conversation, like her needing to leave to meet friends.