Whether your talking to a potential co-founder, or an AI, the game is the same- You want to convince them that you can do great things together, if only you had a little help.
You want them to believe that the goal is achievable, and the rewards are worth it. You want them excited.
No one will ever invest in your idea. Make it both of yours.
is the problem that they don't like the idea, or aren't willing to give up their day job?
if the latter, i've seen pg's essays (either why to not not start a startup, how to make wealth, how to start a startup, whatever) and better yet jessica's book (founders at work) serve as a serious kick in the ass to people in cushy day jobs at ms and google; a few of my friends quit are now in this summer's yc program.
if pitching the idea isn't going well, i've found that giving potential team members problems to solve ("hey, how could this be made to scale to x members" or "what features should be put into the first version" -- whatever, the specific problem isn't the point) gets people going (and as a bonus lets you see if they give smart answers) and 1) makes them feel a sense of ownership/participation in the project and 2) opens the door to nudges like "yeah, these are the kinds of problems we'd be solving!" i've found this kind of end-around persuasion is better than arguing or trying to tackle objections head on (people love to buy; they hate to be sold.)
Colin's spot on. If they are truly co-founders and not just early employees with a lot of stock, they have to believe the idea is just as much theirs as it is yours.
I'm moving to SF soon hoping to team up with some co-founders. But I'm not going with any preconceived ideas of what kind of startup I want to pursue.
You want them to believe that the goal is achievable, and the rewards are worth it. You want them excited.
No one will ever invest in your idea. Make it both of yours.
-Colin