I don't give out a lot of compliments in general. But when I do, I've had better luck complimenting people on appearance when it's stuff they obviously chose and put effort into: a haircut; tattoos; choice in clothing. Few people like to be complimented on stuff they didn't do anything for; many people like to be complimented on stuff they did.
(If you try clothing, though, be aware that "nice top" is likely to be read as "nice breasts".)
There's a lot of data and research on what makes people successful at online dating, but I don't know anyone who actually tried to wholeheartedly apply this to themselves. I decided to be that person: I implemented lessons from data, economics, game theory and of course rationality in my profile and strategy and OkCupid. Shockingly, it worked! I got a lot of great dates, learned a ton and found the love of my life. I didn't expect dating to be my "rationalist win", but it happened.
Here's the first part of the story, I hope you'll find some useful tips and maybe a dollop of inspiration among all the silly jokes.
P.S.
Does anyone know who curates the "Latest on rationality blogs" toolbar? What are the requirements to be included?