I saw this somewhere here on /r/seduction, but I want to repost it because it's incredibly useful. I've used this on OKCupid, by text message, and by email, and it works more often than not. (I think I've used it six times now and failed to get a response only once.) If you've sent a message and haven't got a response in a reasonable amount of time, try sending something like:

Hey [name], if I were giving grades in [texting/email/OKC responsiveness], you'd be getting an F.

Because I teach, I modify it a bit, to something like:

Hey [name], I usually give grades in [subject], but if I were giving grades in texting instead you'd be headed for an F.

If your correspondent is interested in you at all, he or she will almost always respond right away with an apology. If she starts flaking again and you aren't ready to next her, you can always jokingly allude to her grade again.