How we change what others think, feel, believe and do
Set a deadline by which the other person has to decide or act. Make it clear that this is an absolute time by which they must do what you want them to do.
As the deadline approaches, increase the emotional tension, talking more about what will happen if the deadline is missed. This may be specific and threatening actions or vague and disturbing hints.
Use things which cannot be challenged, such as contract completion dates, demands made by senior people and so on.
I must have your answer before we leave today.
I am talking to Steve later. He will want to know what we have agreed.
The product will be released at the end of the week. If you can't deliver by Thursday, it will be too late.
Hurrying people up reduces the time they have for reflection and considered thought. If you can occupy them with worries about what may happen if the deadline is not met, then they will spend less time thinking of objections and counter-arguments to your suggestions.
Deadlines can easily be challenged, but it is surprising how often they are not questioned.