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1. Give them some time. Don't stare them down or have them sit in front of you "until an idea comes out". Give them anything from five minutes ("I am going for a break and will be back in five minutes to give you some time to think") to one day ("Let's talk about your ideas again tomorrow at 9 AM"). This releases the pressure of having to come up with something right now or else ....
2. Ask them for three. If you ask for one idea, this is often interpreted as if there is somewhere one correct answer, that you expect them to come up with. This increases the pressure and often blocks people from sharing anything. Asking for three or five sends the message that there is no single perfect answer.
3. Approve right then and there. Experienced managers often have the tendency to take someone's idea or suggestion and modify/improve/complete it. This sends the message that the manager anyway knows better or more. Refrain from this temptation. Approve their idea, or part of their idea, and encourage them to implement it and get back to you in x days to discuss the result. The ownership and learning will be far more powerful.
We all know how to think but sometimes, a bit of help is needed !