Obviously, there is no possibility to include in any contract that you own your employees ideas. Ideas are in the head of people, and if they don't want to share them, you have absolutely no way to force them to do so, even during work time (in fact the only method to achieve that is called torture!).
I'm not a lawyer, but I'm pretty sure that in your contracts you can only include that their actual work (production) is owned by the company.
So you're on the right track speaking about incentives. Let's check what can motivate your employees to give their ideas to the company.
First, are your employees happy to work in the company? This is the first step. If employees are not happy (because of low wages, bad work atmosphere, and so on) they won't be very motivated to share their ideas.
Second, do you have incentives to reward them for the good ideas they share with the company? It may be bonuses, gifts, promotions, week ends in in a great place — there is plenty of possible bonuses that would motivate them.
Third, do you authorize them to promote their company work? Do you credit them for the ideas they have shared? If they know they will be able to show to the world the good work they done in the company, they will be more motivated to give their ideas to the company. I know however it's not possible in all cases, given some of your company's customers ask for confidentiality about credits.
Fourth, do you give them some time to work on their ideas on work time? If they are all the week working on your customer's projects, they will necessarily take time to develop their new ideas at home, not at the office. Think about Google, giving their developers one day per week to work on personal projects. Numerous Google products have been created this way (will add references about that tomorrow). This is a tech company, but this way to let people be creative can be easily transposed to creative ones.
Anyway, it's a common problem in any creative companies, and there is no easy way to solve it. And it's not necessarily a bad thing, world is moving, people is moving, and you won't force people to stay in your company and continue sharing their ideas. You'll have new employees, maybe more motivated, willing to share their ideas, at least for a while. Sorry for this philosophical thought, I know it's not the place, but I think you have to take this in account.