Used to be that when I write sales proposals, I attached a period of validity for the sales proposal. For example, 2 weeks.
The problem appears to be that there's no incentive for customers to act immediately. Hence, I've decided that in my future sales proposals, I'll attached further conditions. After the period of validity is over, for every two weeks, the price goes up slightly, so that customers will have incentives to act fast.
The earlier they placed their confirmation for a proposal, the better it is for both parties.