I've always just asked how they are, commented on their appearance, and/or thanked them for replying.
Most get back to you with some sort of reply, whereas others you'll not hear from again.
As said earlier in the thread, women are different. Some model type may spend the night for £100 with a 60yo man, 3x her age for whatever reason yet a 4/10 won't acknowledge you or even accept £300 for a couple of hours.
I've gone to efforts of tailoring responses to their profiles / pictures and still had short responses that may completely avoid what has been said. Sometimes their profiles are complete bull like ours and they've zero knowledge of things they apparently love the most.
I've had times in the past where 1 or 2 out of 50 have bothered replying regardless of what I do, say, photos I have up etc.
Sometimes it's a case of thinking outside of the box and rather than telling them they look beautiful in their picture, which they may have heard 70 times today, maybe the location, jewellery, tattoos, dog in picture 3 etc. are better places to start.
If the numbers are there, sometimes being straight to the point works, assuming SA doesn't ban you.
Some women want to know you're serious and not playing around, so by putting some cards on the table, you're showing her that you're not a timewaster and it's worth investing time in you (which in girl terms, might take days or longer even though she's online).
Also mentioned earlier, play around with ones that are of less interest to you. See how things work out with them, fine tune things, maybe collect numbers for later on or as a backup plan. The girls that seem less desirable can often turn out to be a far better experience than the 9/10 hot model type you're desperate to shag.