Explain. Explain. Explain. People need to know how you work, so explain the process. If you provide an initial consultancy meeting, take notes, return with a customer solution then make the sale, then say so. People like to know the process.
Deadlines. If you are making a special offer, make sure that you give an end-date. This creates a stronger response and stops people from making claims after a sensible period of time.
If something is subject to availability or while stocks last, say so. This means that fewer customers will be disappointed and more will respond fairly quickly.