You need to make it easy for everyone in your organization to reach you and to communicate within the origination. You need to be able to have ways for people to front ideas, discuss ideas without needing to go through supervisors ect.
Regular polls, contests and awards for fronting for example the months best idea will help the staff to brainstorm here and there.
For your employees to be innovative I think they need good forms for communication, and be able to post suggestions to the org. anonymously.
Every month or so you should have a dedicated set of a couple of people to go through every idea and screen them. If you promote prizes and awards for the best ideas I think you will promote an innovative culture in your organization.
I also think its a very good idea to actually spend some resources into this. Innovation is extremely important and the better ideas should be given a explanation if they cannot be realized at this time in your org. Many of the ideas that will be presented to you will most likely be good, but not properly thought through and many will fail to see the big picture, which is why its a good idea to explain why this will be very hard and resource demanding to put through.
Edit: Be sure you dont dismiss all ideas upfront just because they are costly in resources to put through. If you decide that an idea might be good, but might be costly, that idea could be worth to look more into. Ultimately you want your business to be ready for the future, and be able to expand and grow. This wont happen in the long run if you dont have innovation and are able to put to life some of the ideas.