I just pulled this off successfully a couple days ago, so here's my method:
1) Invite her to meet with you, somewhere low pressure, like a coffee shop. Preferably in a nice area of town. DON'T frame it as a date, just ask if she would like to meet you.
2) Meet, get her some coffee, sit down and talk. Stay very relaxed, and talk to her like an old friend. Be genuine and tell her about your life, using plenty of DHV's and all. Not bragging, of course.
3) Ask her if she wants to go on a walk (the reason for the nice neighborhood). Hold the door for her, touch her back on the way out. Any excuse you can for kino is good. Walk and talk for a while. Put your arm around her for a minute, then find some excuse to take it away (scratch your nose or something).
4) Put your arm around her again, and leave it this time. Keep walking and talking like normal, don't act awkward, make it completely natural, like you're still dating. Find a nice setting on your walk with nobody around and kiss her. Then say something like "I couldn't resist." Then keep walking again with your arm around her.
5) Kiss her again later, but make her wait a bit. Plenty of talking in between, which builds some tension.
...this is as far as I got the other night, since she had already made plans for after our date, but here's what I would have done otherwise:
6) Think of some excuse to come to her place or bring her to yours (hers is preferable, since she likely feels more comfortable there). Look at her artwork, watch a movie, whatever.
7) Go to her or your place and start watching the movie or whatever. Make out with her after a bit (again, make her wait for it). Then stop for a minute and talk/watch the movie/whatever. Start making out again, but get into it a little more. Stop for a second, start again. Repeat this cycle, escalating slowly each time.
Have naughty sex. Hang around a bit and talk some more, but not long. Tell her you loved hanging out with her again and leave. 9) Continue doing the same thing whenever you like, and never bring up the prospect of a formal relationship. If you keep doing it long enough, she will at some point. At that point you just have to be honest and there's a good chance your arrangement will end there. Girls always get attached, so it never works in the long term. You either have to agree to a relationship at that point or find someone else.