Yeah, man you can do it right out of the blue. Pout your face, walk up to them with your arms opened and ask "can I have a hug?" Don't do it too often, more than once a week is probably too much because it's kind of a wierd thing to do, but it works. Last year the first thing one of my friends said to a new girl in the class was "can I have a hug?" He didn't even say it seriously and she was happy to oblige.
And if she says no, then she's obviously not interested. She's also a total garden hoe.
If you're walking in the hallways and you see her but she has her back to you, touch her one way or another in a friendly way, squeeze her or something somewhere on her back or her side (lightly...) and then walk away without looking back. I did this to a little freshman and two weeks later she's into me. Trouble is, I'm not really into her.
Most girls are ticklish. Some are more than others. Some are much more than others. And if you tickle them, chances are pretty good that they're going to tickle you back.
Yeah, all I can think of now.