1. When You Create Your Twitter Handle Consider The Following:

If you are a household brand name, like Virgin America, by all means use it. Followers will be looking for you.
If you are a well known in your category and people are already seeking you out, use your name in your handle. Again, people will want to find and follow you because you've already built a personal reputation offline.
If you fall into the category of virtually unknown, like most of us, consider using a descriptive name in your title, for example @winedinetv. Building my following would have been much easier had I used "public relations" or "social media" in my twitter handle.
If you use your company name and/or your personal name and you don't have a significant offline following, describe what you do in your profile.

2. Fill Out Your Profile Completely

Think of this as your introduction to your followers. If you were at a networking event you would introduce yourself and at some point get around to telling people what you do for a living, your philosophy on life, the books you've written; whatever you want people to know about you.

3. Post A Photo Of Yourself

This proves you are indeed, human. Leaving the Twitter icon up looks like you might be a spammer (this is the equivalent of talking to a wall at a party and no one wants to relive THAT experience). We found when we had our logo posted we did not get as many followers. When we posted my personal picture, the numbers of followers increased greatly. Again, going back to @winedinetv, they use their brand name, but put pictures of themselves up on their page.

4. Do Not Lock Your Updates

Locking your updates tells potential followers that you're not interested in having a two-way conversation with them. We have a client who was complaining that no one was following them, but they had locked their updates. We unlocked their updates (and changed a few other things) and followers poured in.

5. Follow Your Followers

The whole idea behind participating in social media is to have a two way conversation with the folks in your group. If you don't follow people who follow you, it's like you are staring mutely back at someone who ask you a question.
There are a few exceptions and I may be a prude, but there are some people who I don't follow back because of the content that they are tweeting. Some of that stuff I just don't want to see pop up on my screen!
If you don't follow your followers, you may find that they stop following you.

6. Follow Your Follower's Followers

If your followers like your content, their followers are likely to be interested in the same topics.

7. Be Consistent With Your Tweet Content

This allows people to find and follow you based on content. With few exceptions, I consistently tweet about social media marketing, public relations, wine and restaurants. If someone is considering whether to follow me, they will have a good idea of what I'm all about and decide if that's information they are interested in receiving.
If your business is geographically specific, i.e. you run a furniture store in West Hollywood and you know your customers come from a 15 mile radius, you need to mention "West Hollywood" in your tweets as many times as appropriate.

8. Give Back To The Community

It's fine to send people to your website for specials, deals, information etc. But if you only do that, you will earn a bad reputation as a taker, you will lose followers and break the trust of your followers.
Find articles that talk about your area of interest or retweet other posts.

9. Avoid Following People With Locked Updates

Do you really want to try to have a conversation with someone who is already telling you they don't want to talk to you?
I have found that these people are less likely to follow back

10. Tweet (seriously)

I realize this seems obvious, but clients will tell me that they can't understand why no one is following them, but they posted three totally lame (and you know who you are) updates months ago.
You need to post on a regular basis with relevant, helpful information. Except for your parents, few people care that you can't find matching socks in the morning.
We recommend that client's tweet at least every 48 hours.

11. Don't Over Do It

If you are tweeting too frequently, unless you are reporting breaking news, you are probably sending out too much information too frequently that can overwhelm people and turn them away.