Short Answer:
Future Projections and Seeding
Long Answer: Some of us know what these things are, some of us don't but the idea here is to create a fun idea or scenario in her head.
Why should she go out with you, someone whom she met on line? Its a good question from her perspective considering she knows that most guys are looking to get laid and she hears all the horror stories from her friends and the news etc.
Well, first point is...
DON'T BE EVERY OTHER GUY (AFC).
Sounds obvious but, you'd be surprised how quickly you can fall back into that. I don't care how funny you are or how great your disqualifies are etc, You have to remember she may read your message every 3 days. Its not like in person. It can be going great but the minute you slip into AFC and she hasn't heard from you in 3 days, you become an AFC to her. Keep the conversation original, funny, unique.
second...
ALWAYS Accept what she gives you.
You may be role playing, talking about whatever... you might say, remember the time in Vegas we were walking the streets like we owned the place, blah blah blah. She might say, Yeah and then we jumped in the fountain and splashed each other. ~Your response will NEVER be No, or alter it in any way~ Instead, you should reply with "Yes, and".
"Yes, and" works magic! What do I mean... well you are accepting what she gives you. jumped in the fountain... you say, "Yes!, and we had that guy take our pic and he tried to run off with your camera!"
See what I mean. Accept what she gives you and add more detail.
You role play by saying shes your maid or whatever and she says Only if she can wear high priced stilettos. You say... Why Of Course! and Ill get you the new Feather Duster 4000 series.
Examples are endless.
Thirdly...
SEED and FUTURE PROJECT
This means what it says. Plant a seed. One I use is, at some point Ill ask if she likes coffee and she'll say yes and rattle off something about a caramel frap, whip, latte, mocha... whatever, lol
I'll say I drink my coffee black (which I do). Continue your conversation as normal. Meanwhile research a cool coffee shop in your area and say something like this...
I know this cute little coffee shop in (city) that has overstuffed chairs, an upstairs and an ever-changing art show on the walls. Plus, super yummy food and great coffee. I drink mine black but, you can get yours with all the Fu-Fu stuff on it including whipped cream. That way if you're not nice, I can fling whipped Cream at you. haha You do like a good food fight don't you?
This works great! you didn't technically ask her out but, she is imagining having fun with you.
Fourth...
GET HER NUMBER
You can ask for the number with something like this...
So Let me ask you something. . .
"Can I..."
Hold on a sec....*ahem*
"Can I have..."
Heh heh....just a little nervous, that's all....
*clears throat*
"Can I have your number!?!?"
Seriously though, as long as you promise you're not a crazy internet stalker who only wants me only for my body, you should give me your number, cause I'm running out of things to type. I mean, i'm not a book of sarcasm. I'm better at it in real time, lol.
And as a special bonus, we get to be text buddies!
99% of the time you'll get the number.
Fifth...
TEXT HER
remember your conversation from on line. DO NOT text her and say "Hey it's "Joe" from My Space". If you do this you have just reverted your relationship back to the beginning. (Introductions)
Instead text her something from the conversation you had to put her in the same on line mind set in your new medium (Text)
Hey, I saw this girl drinking coffee today with an ENORMOUS amount of Whipped Cream and I thought of you
Sixth...
MEET UP
Meet me at 7:30 tomorrow at (Cool Coffee Shop) for a Java Jamboree and some stimulating conversation.
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Hope this helps a little.
Interested in your feedback

Cheers,
~GM