Make sure you're competent.
Incompetent salespeople think they can wing it, and live off their charm.
Prospects see right through this, but resonate to salespeople who are sincere. Know your prospect's business. It's your job to know about prospects' business before you take their time, either on the phone, or in person. When you go in to an interview knowing how your prospect functions, you can easily explain how your product or service can benefit him or her. Be prepared to demonstrate cost savings.