The most common mistake that companies and individuals make is talking about their own emotions, like "We're happy to announce..." or "We're excited at the opportunity..." Customers don't care how you feel. They care about how you make them feel and people feel good when they talk about themselves. Let your competition have the "Me Me Me" messages.
When it comes to selling, there is no more valuable piece of information than what you learn when you ask: "Why didn't you buy?" Way too much sales energy goes into replicating success, when the real issue is avoiding the errors. Correct those errors and your competition will keep spinning their wheels.
It's amazing how few companies "get" this. Never ship a product or provide a service without providing at least just a little bit more than the customer expected. And it's got to be more than a coupon for future purchases. Include a free gift. Take an extra 5% off. Even a personal note counts, if it's sincere and not boilerplate.
Selling is like chess; if you want to win, you've got to think five or six steps ahead. If a customer is important to you, you should know--even before the customer knows--how you'll be able to help them in the future. Most importantly, think at least a couple of steps ahead of your competitors. Checkmate!
This is a very basic rule, but few people follow it when it means losing a sale. Instead, sellers tend to convince themselves that the customer should buy, even when the seller knows, deep down, that it's not the customer's best choice. If you make a sale, you've got a customer. Forego a sale (when appropriate) and you've won a friend.
I get sales emails all the time that contain obvious errors in the SUBJECT line, let alone the body of the email. A good copyeditor--a human being, I mean--will ensure that what you've written actually makes sense. For my non-blog articles, I use Pure-text.net but I'm certain there are other companies that also provide this essential service.
When you find customers for your customers, you're no longer somebody who's selling--you're now part of their team. Finding customers for your customers is, in fact, the holy grail of customer relationships. There is nothing in the world that creates competition-busing customer loyalty even half as well.