I'm in the same situations, but I've found.. not too longer texts, try and slip a joke in there, try and make her laugh if possible, and just keep her attention.
I wait for her to text me rather than text her and also don't agree with everything she says, I've found that not so much making fun but like just making a little joke out things is good.

Make jokes about yourself as well, don't make it feel like you're picking on her.
And when asking her out have a plan in mind, as you'll nearly always get the response 'i don't mind' from a girl over text if you say do you wanna do something soon, or 'i'll let you know' is pretty much code for, i can't be bothered.
Say: "Hey, I was planning on hitting the cinema tonight, but my friend bailed on me. Fancy tagging along?" Or "I've got 2 tickets for the movie tonight, wanna be my +1?" Something along those lines... I'm not gonna pretend i'm and expert! lol but hope this helped!