Your Career And Money:

Sales training programs strongly make this point. Sales are frequently lost because the salesman cannot shut up after he has made the sale. It seems like such an obvious point, but it is shocking to me to observe that people can't stop talking.

If you think you have won, get a commitment (preferably in writing) so everyone is clear that it is the "done deal", shake hands, and move on to a new subject, or leave. Don't keep rehashing the benefits, costs etc. Once the person has said "yes". Such delays give the other person a chance to reconsider their position and possible back out of the deal.