1) Offer money
Offering money upfront is best. If you say "you'll get a portion of the revenue" that isn't very appealing, because even the most simple games take a good amount of time before they're ready to be sold and there's no guarantee that your game will even get that point anyway. No one wants to work on someone else's ideas for free.2) Contribute something more than just ideas
As others have said, we all have our own ideas and if people are working for free, they'd rather work on those ideas. No matter how good your ideas are, telling people "these are my ideas, now go make my game" is a very bad attitude to take. If you retain sole control of the game's design, you need to be working as well. If you have no talents as a programmer or artist, you can contribute by managing the game's website, being a play tester, managing the project and organizing people, etc.3) Provide a core vision, but allow others to share their ideas as well
There's a lot more motivation if people feel like they can add their own input into the game design and get their own ideas improved for inclusion in the game. Let the game design be a team process, not just an individual one. Try to put together a core design and propose an initial set of the main features found in the game, then once people start joining your cause, allow them to comment and suggest alternatives or even remove existing features if they have a strong case for why they feel it would be bad, or want an alternative that they feel is better.
Also it's important to note you need to realize that if you have no technical skills or knowledge, you usually will not have a clue about how difficult (or impossible) it is to add certain features. And you also can't fully know if an idea would work well until you see it in action (some of my ideas have worked out better than expected, others were so bad that we tossed them out).
When I started my project years ago, I was already a capable programmer (and have improved very much since then) so I had something to offer (#2). And I welcomed people to submit their own ideas, especially during the initial period when the project was forming (#3). Although it was hard for me, some of the early ideas that I had for my game were voted on and rejected by my initial team, and I had to let go of them even though I was really excited about a few. But I think that was for the better.
The core vision that I laid out for our the game design (below) has remained. It has served us extremely well. Whenever someone proposed a new idea, we check it against this and if it is in gross violation of these principals, it is tossed aside.
- Create a role-playing game, free to the public, which may be enjoyed by as many people as possible. It will be playable on a wide range of computers from 1990 era PCs to today's, and on virtually any user operating system from Linux, to Windows, to Mac OSX. This game will also support multiple languages so that players from the world over may play it.
- Design the game such that the major focus is on gameplay and story, not advanced 3D graphics and physical simulations.
- As much as possible, remove the tedious, meaningless, and micromanaging aspects of many historical and modern RPGs.
- Require a high level of strategic thinking and planning from the player, and less mindless "button mashing" found in many RPGs.
- Make the source code and documentation to our game engine freely available under the GNU Public License, so that other game developers may absorb what we have learned and use our code to expedite the production of their own games.
If you want to attract people to work on a project formulated around your ideas, there are a few different ways to do this.