Start with advertising what exactly sets you apart from your competition. What makes you different? If you have the same products as your competitors- consider what can make you different. Is your return policy better? Do you offer your customers better perks? Find your angle.
You should also know who you need to sell to. I once knew a guy who performed horrible as a sales person. It wasn't that he couldn't sell, it was that he didn't know who to sell to. Make sure you are always pitching to decision makers. Don't waste time pitching secretaries or even company owners. Pitch to the people in charge of buying and making decisions.
The last thing you should do is deliver the goods. The key to sales is repeat sales. If you do what you say you are going to do, are friendly and prompt- you should have no problem gaining your customers' loyalty. So, make sure you thank everyone for their business and provide the products and services when promised.