The question is, how do I escalate.
The answer is, escalate.
But of course, that goes in a circle, so let's hit some specifics.
You open, all is well, conversation flowing. The problem is, then it stays there. You need to move to the next part of the process. The next part is to make it sexual. By that I don't mean you hop on her. I mean through tone of voice, topic of conversation, the way you look at her, how close you stand to her, the amount of natural kino, you let her know that you are interested in more than just a friendly conversation.
Escalating sets you apart from the average guy she just talks to and sees as a friend. It lets her know without coming out and saying it directly that you want to take things to the next level. She then knows what you want and can react accordingly, either by running screaming for the exit (in which case you just avoided wasting time talking to her for hours and hours) or by staying in the interaction and hopefully giving back indicators of interest. Or she stays in but starts shit testing you, which means she's not sure. She wants to make sure you are the person you're presenting yourself to be. If the shit test knocks you off your horse, it's probably game over. If you show no reaction to it and just keep moving forward, if you turn it around, if you handle it in a confident way, then it's game on.
I hope that helps. If you want something more specific, there are many "techniques" out there for escalation. One is to hold out your hand and see if she takes it. If she does, that's obviously a good sign. You both know there is more going on than just a conversation, but you haven't pushed too far too fast. You are leading, you are confident, you know what you want. That's all good stuff.
Good luck. This process has many stages and mastering each one at a time is a good way to go. Keep moving forward and you will get where you want to go.
-Zen