Include an invitation for them to respond (eg, asking them a question). these work best towards the end of the message
My god I cannot agree with this enough.
That being said, the way I send a first message is not a copy / paste thing, but i do slightly disagree with the first reply because I do follow a sort of magic formula so to speak.
1) I always say hello, how are you, nice to meet you, or something along those lines - and specifically use their name. I know it's not really 'meeting' someone but it does make a message feel more personal feel which might make you more likely to get a reply.
2) "I was just reading your thread and really liked..." basically just to let them know it's not a cold call message and that you have seen what they wrote.
3) "What I was most interested in was <insert specific plot, pairing or idea here>" - Again, showing you read it, but also prevents you from seeming vague and needing your potential partner make all the decisions for you, which most people dislike.
4) Make a suggestion regarding something, whether it be an addition to the plot, something about the characters or the setting and ask for their opinion.
5) Kinda like the same principal as a cover letter when you apply for a job...I mention that I'm looking forward to hearing back from them, even if the answer is no to my suggestion(s).
I find that falls in the sweet spot for not too short / unimaginative, but equally not an essay that completely changes their idea or goes off on a tangent that they might not be interested in.