How to get more twitter followers?

One technique to that is to follow niche-targeted people and then tweet regularly but not spamming. You should tweet the ones relevant on your niche as well so you'll get more retweets as possible that'll give you chances to get noticed by all other networks interested in your market. Posting 5-7 tweets daily is essential.

How many are you following?

In order for most people to follow you, you must be part of their network. Find people that are interested in similar subject as you and then follow them.

And just as here provide valuable feedback and conversations when possible.

Michael

It's really not about the number of followers that you have. Instead think about trying to find influential people to interact with on twitter.

Having something mentioned by an influencer on twitter is much more valuable than you spamming links to thousands of followers.

Stop looking at a follower count and start engaging :-)

In order for most people to follow you, you must be part of their network. Find people that are interested in similar subject as you and then follow them.

I've been in Twitter for long time now and there are lot of ways to get followers fast.
But before answering your question I just want to know why you want to increase the number of your followers.

Great advice Brad and just to follow up, Gary Vaynerchuck says this from his talk at SXSW back in March:

Unless something has happened to Twitter your ratio is way low. I have found if I follow 1000 people--I get about 300 to follow me. Once you do that get rid of the 700 who didn't follow you. They are no good to you. Tweet and tweet and tweet. No spamming. If those 300 don't respond--start weeding them out. They just are not that much into you. Twitter is a lot like writing good headlines for an article. Practice each tweet. Don't just push junk out there. Work at each 140 characters as a life or death situation. Make it valuable. Twitter is a modern day challenge. Make every third tweet a personal statement about something like the weather, events in town or what you had for dinner. The next time you get a tweet from a marketing guy--send him or her a direct message. Ask them how successful are they with their product. Ask engaging questions. Who knows you might make a friend. Learn to make people want to respond to you. It's fun--but it takes time. Perhaps, too much time.