Be Clear with Your Meanings
We've all gotten used to LOL'ing and IDK'ing, but the last thing you want to do with a girl you are potentially going to be in a relationship with is send mixed messages. Sticking with standard abbreviations that are universally understood by anyone with a cell phone is generally OK, but don't overdo it with anything that might be ambiguous. Having a limited number of characters to work with can also make it tricky to send messages with the proper tone, so choose your words carefully, lest you scare her away.
With phone calls among those courting and getting to know each other, there is an unwritten etiquette that says that phone messages should not be returned too soon, lest the person returning the call seem too desperate. But because everyone usually has their cell phone with them and because the norm has become to respond to texts immediately, the response time on received texts is much shorter. Waiting for two days to respond to a girl's flirty text is far too long; if you are interested and want to be flirty in return, you should respond within one day. Note: If you receive a voicemail you should NOT respond with a text. Responding to voicemails with texts will send the message that you are not that interested.
After you've been on a first date with a girl, it is a good idea to a send a text the next day, telling her what a nice time you had. Even if you didn't have a nice date, this is simply polite. If you did have a great evening, with lots of sparks, it is a good idea to send a cute, flirty text letting her know how happy you are with how things went and that you look forward to seeing her again. If she felt the same, it is a nice way to boost her confidence and let her know that you are of the same mind and that she can look forward to date number two. In this post-date text(s), you should point out things you liked about the evening, and take the time to compliment her appearance or other things that roused your interest in her, specifically.