If you want to control your idea, then perhaps you need to think about the project from the architect's/project-manager's perspective. Put together a draft design, then start writing story/feature descriptions. Break the problem down into many simple manageable pieces, then ask for people to contribute to building a few elements without them needing to understand the overall design until you are in deep enough to maintain the creative control you desire. The pitfall of course is that people like to be involved in the projects they participate in, and you lose the opportunity to get valuable advice from your colleagues.
If your goal is to make money out of this idea, you need to think about whether the idea in and of itself will satisfy that requirement, or if there is an ancillary source of revenue related to the idea you wish to pursue. Each option allows you to provide a different incentive or motivation to anyone interested in joining your project. And regardless, you'll still need to plan out how you want to do things when you bring others on board.
If money isn't the object, then it won't matter who you get to help, because trust would be the greater issue. Of course if you know someone you absolutely trust who will be willing to join you in your venture, then perhaps a business partnership would be a better way to approach this.
At some point, if you wish to have others involved in your project, you are going to need to actually let them in on your idea. As to when in the process that is, that really comes down to your own comfort level in that regard, and as such, the strategy you implement to allow you to develop your idea will be influenced by the level of trust you are willing to give to your colleagues.
If you decide that an open-source, or a similar closed-source/open-development model will work for you, then you'll want to build a community around your idea, and allow the community enthusiasm for the project to drive things forward. Defining a project on a website such as GitHub, or SourceForge would allow you to state clearly that you are the source of the idea and the project leader, while still allowing you to invite others to be contributors to your project. In all cases however, unless you have a lot of money to burn, I'd avoid using contractors as they won't have any emotional investment in your project, and will likely provide you with a result that will leave you feeling as if you should have simply done it yourself.
Good luck with your venture! :-)