I know that shorter messages are better in terms of reply rate. The optimal length is something like 50 characters. Characters, not words.

A great piece of advice for online dating is to stand out from the crowd. So greetings like "hello" and "hi" are very common. They do less well than things that are a little bit quirky or a little bit weird, like "howdy" or "holla." The rarer your salutation, the better it does, in general.

Women are much more discriminating. I think both types of people are equally interested in having an attractive partner. But women essentially give the thumbs up to only half as many guys as guys giving the thumbs up to women.

Going to a site with more users seems obviously better to me, but at the same time, having a bland, middle of the road profile with bland, middle of the road pictures seems like a bad strategy to me.

You take a picture of yourself in some exceptional situation - skydiving or whatever. People always post those photos because it works - you're saying something about yourself that begs a conversation and that's what the users are there for.

Kind of paradoxically, men are very open minded or very even handed with their votes of women.

Use correct grammar and punctuation. Do not use net speak, like WOT, W-O-T or U. Those messages get a lot lower reply rate.