How to Get a Girl in Your Aikido Class to Like You
Talk to her before and after each class. If she's just sitting in a chair, or standing somewhere, go up to her and talk about something interesting. But make sure she isn't already talking to someone else.
Ask her for her email address or her IM. Do this at least the fifth time you see her if you talk to her a lot, and if she asks why, you don't have to tell her it's because you like her, just turn it around and ask why not.
Don't make fun of her if she does sloppy push-ups, or a bad counter. You can do it once in a while, but not every time you go. Try not to be too patronizing, but it could give you the opportunity to show off and help her with her technique. However some girls might take offense to any form of criticism so be careful.
While sparring her, if you get to do so, don't go too hard on her, but don't go so easy that it's obvious. Remember, she IS learning the same techniques as you, and may well feel you are insulting her abilities by going easy on her.
Compliment her on her Aikido skills after class. Girls love it when they get compliments. If she's more advanced than you, ask her to help you with a technique. It'll give you the opportunity to talk to her while flattering her skill.
Saying Good bye. When you're leaving, or when she's leaving (which ever is first), say good bye to her. The best way to say it is see you later, so the girl knows that you actually want to see her later.
Make sure you don't have anything stuck in your teeth when you talk to her.
If there are any awkward silences in your conversations, quickly say something funny or interesting.
If you're talking to her, stare into her eyes and don't look around in different places in the room. It'll annoy her.
Check if your dojo has any rules about dating someone from the class. Some have rules like this to prevent both sexual harassment and interpersonal drama from cropping up in class.