You don't want to be spam.
I've been consistently tweeting since February 2008, and I've seen it all. Here are the ways to win.
Four Simple Ways To Win At Twitter:
1. Have a fabulous product
2. Have fabulous customer service
3. Connect with customers
4. Give something of value to your followers: humor, enlightenment, ideas, etc.
(More on #3 and #4)
Let us know who and where you are. On Twitter, you don't want to be seen as a "corporation", but as a friend. This means, your followers should know who they are talking to. Once a follower can "picture" you, one of the walls has been taken down.
Tweets shouldn't be all business, all the time. This is boring. Instead give us some insight on what it's like to work at your company, what you do for fun, anything that might make a connection.
Engage with others, stop trying to close a sale all the time and just ask questions, get to know what your customers enjoy. This should not be a one way conversation.
And finally: do not fall prey to the thought that you should be tweeting all the time. Do what works best for you and be consistent.
Homework: Study @Zappos and apply the principles you see.
And finally (yes, I know I already said that) HAVE FUN! If you aren't, your followers aren't either.