Figured this fun little open technique out today.
If you're walking behind a cutie and there is a door somewhere in the near future, drop back a few feet - just enough so that she knows you are there, but far enough back that she has to hold the door open for you.
When she holds the door open for you, she will be waiting for you, looking at you square in the eye.
Smile warmly. Open her up with a friendly "Oh thanks. You're the nicest girl i've met all day!"
Don't do this sarcastically. Be genuine and playful. It's a great way of making the approach seem natural.