These are some general hints I've found work for me:
I have found that putting attractive pictures and colourful lettering always helps get people interested in at least reading the idea.
Obviously, your title is the thing that will grab people's initial interest, make them open the thread. Obviously it has to relate to the content in some way, but to use my own current group as an example: if I'd called it "Dressing up as a horse", it wouldn't work anything like as well as "The Pony Palace"!
Make the thread readable - People hate reading a wall of text, so break things up into smaller paragraphs. Put large volumes of information behind spoilers. When you create your thread, go back in and reserve three or four posts before anyone else has a chance to even see it, and split the source material liberally over those posts, in logical sections.
Make sure your thread is aimed at your target audience - I know it sounds obvious, but if you want to set the story in the Stargate universe, there's no point posting a pic of a Federation Star Ship. (No, I haven't seen that - not yet anyway, it was just the best, worst example I could think of).And then there is Moraline's very helpful thread
. The checklist given there can be used as a prompter to make sure you have all the information you will need, or you can actually include the list itself and fill it in - probably best to do so under a spoiler although I have left it sitting out in the open.
Another helpful thing is to have an open sandbox. If you need ten players for a very specific story, chances are even if you get them you will loose half within a week of starting and your game will fold. So allow for players to enter the game after it starts, and to drop out without killing it - a statement saying that players who drop out will have their characters taken as NPCs to remove them from the game is useful!
alxnjsh's suggestion is a wise one. PM people you know who think might be interested and ask them, point them at the thread. The worst that can happen is they say "Sorry, not for me". I have a small group with three other players who are all friends that is chugging along nicely.
But the biggest thing is to have patience. Never post an idea thread and expect to get interest the same day. It could take anything up to a month to get enough people, depending on how many you want. We all think our ideas are the best and should get instantly hundreds of others interested - I know I do! But in practice it rarely works like that. I have posted ideas threads that I thought would get hundreds of players, and got none, and I have posted (so far) three ideas that I thought would flop that actually took off.
And one idea that I was pretty sure would fly and is doing so.
Lastly, it's all very well for me to post an idea that I want to run a group game based on the books by C J Cherryh, but if no-one else on E has read those books I'm going to get a pretty low response, unless I can somehow open the idea up and make it accessible without forcing people to read the books.
I hope this is of some help to people?