It's a combination of common sense, manners, charm, banter and honesty really you just have to strike the right balance.
Read a girl's profile properly, if what she has to say about herself on her profile doesn't engage you in any way, just don't bother.
The way Tegan and I started messaging was because I saw we were both chefs (she's just a bit of a looker too

) so we started off by talking about that, then the conversation just flowed and it was all good.
Don't copy/paste message. You have to prove that you're interested, basically.
If you don't get a response, don't push it, move on. "Hey I msgd u ova day n u dint ansa y not?/" messages isn't a good look. You have to remember that on PoF, you're selling yourself, not vice versa. Girls get bombarded with messages so they really do have the pick of the crop, so make sure yours is prime.