Well if I were you I would start with slow and shallow but as you and your girlfriend get into it start to go a little faster don't go to fast. As long as she's moaning you know your doing something right. And always keep your penis in mind.
Personally, Long, deep strokes feel the best. If you are not pulling almost all the way out each time, it is more difficult to really feel sensation from it. Just moving slightly back and forth while you are inside really does nothing for me.