One of the most important things you can do to be a more effective salesperson is to have a powerful, commanding voice. Like a firm handshake, an impressive vocal presence subcommunicates power and leadership and will cause prospects to be much more willing to buy from you.
Source: Author Frank J. Rumbauskas, Jr.
Practice speaking louder in your everyday communications. You don't want to yell or strain; instead, focus on speaking from your core, your abdomen, which will result in the commanding voice you need to have to be effective.
Imagine a general who speaks powerfully, but without yelling or straining. This is what you should strive for. I achieved this by simply talking that way all the time. An added benefit is that you will automatically become an excellent public speaker by having this talent, which you can then leverage into more sales by volunteering to speak at networking events, chamber of commerce meetings, and other "target-rich" environments.
It will also be a necessary skill should you wish to go into sales training or public speaking later in your career, a choice that is available to all successful salespeople.