The most popular method is to find people and follow them. Following many people results in you having more followers. It sounds easy but remember to not only follow but also provide values, otherwise people will unfollow you. Following to many people might looks suspicious and scammy, so make sure the number of people you follow is reasonable, otherwise no one will follow you as they might thing you are a scammer.

Unless you are a celebrity or famous on internet for whatever reason, you will have to work a bit to get more twitter followers.

Tip 1- Get followers by following, but take it easy. It is not a good idea to follow hundreds of people a day. Do everything top keep your following /followers ratio near 1-1. Let’s say you follow 500 people and have 500 followers, then it would not be wise to follow 500 more in one day, rather in a week or more.

Tip 2- Provide value, share interesting stuff, give relevant information.
Share links to interesting things on the web.
Participate in conversations.

Tip 3- Complete your bio and provide a picture. Your empty profile will make many people to turn away and not follow you. Add a picture of you and write a few things about yourself. People like to know who they talk to.

Tip 4- Start a blog or create a website. People will be able to know you better if you provide them with a link to your website or a blog. Your short Twitter profile can't show your personality as detailed as your blog.

Tip 5- Remember to reply to your followers and re-tweet interesting stuff. They will do they same, and you will be visible to their followers.

Tip 6- Do not miss a Twitter party. Go to every Twitter meetup organized in your area. You’ll find many new followers there.

Tip 7- Get listed in as many Twitter directories as possible and share your username in places such as social networks, forums and blogs.

Tip 8- Be unique, be you. Say what you want to say and what you really feel. An interesting opinion will surely get retweeted and your profile will appear to others. Be an individual, personalize your profile, change colors and background.

Tip 9- Tweet regularly. Do not worry, you do not have to tweet all the time, but try to do it every day. You will lose followers if you do not update your profile frequently.

Tip 10- Last but not least, balance all of the above. Avoid talking about yourself all the time as it will make people bored and tired of you and they will stop following you. Good idea is to post 1-2 out of 10 messages about you and the rest to provide value to your followers.

Balance is crucial.