How we change what others think, feel, believe and do
Say that something must be done by a certain deadline or else the deal is off.
Make the deadline in the near future and such that the other person will panic.
Explain how, due to circumstances beyond your control, if agreement is not reached within this short timescale, you will be unable to find a satisfactory conclusion.
Show them what will happen if the deadline is not met.
The project milestone is next week. If this report is not ready by then, it will slip at least a month and it will be your responsibility.
Prices go up at the end of the week, sir. You haven't got long.
If you're not in bed by ten, you will not wake up in time tomorrow.
Constraining the time in which people have to make a decision forces them to consider the other side of the deadline and what would happen if it is not met.
When there is some action to be completed, the other person will be focused on all the things that have to be done between now and the deadline.
Hurrying people, especially if it panics them, has the effect of reducing the rational and reflective thought that they put into the process and thus makes them more likely to agree with you.