You can consider setting up automatic following back of everyone that follows you. I am sure many follow me for just that reason. I regret to say I only follow those I need or want to. This may hold back the growth of my following, but it does mean the response rate to content is vastly improved.
Jump on coat tails of others.
You can always make content that then associates you with larger twitter users, and maybe share content that they will then retweet. This is a good self-serving tactic for you both, but make sure that they are relevant to you, otherwise the followers you acquire could be short lived.
Don’t ignore the option of Twitter ads. You need to try them for your niche before you can really make a comment on whether they work for you or not. You may want to repeat the exercise again in the future at many think that traction to your first 1,000 followers is the hardest.
Practice strategic following then connect
Taking on key influencers and then interactive with them by retweets or quoting them will attract their attention and that of their followers. They may well then follow you to see what else you have to offer.
Avoid blatant promotion
I have heard various rules but all follow the idea of a couple of retweets, a couple of interesting tweets on your own and then you can through in a blatant plug for your service, or just a reminder of why you are there.
Don’t use automated messages via an auto-responder
They stick out half a mile and show you don’t care or take personal charge.
Reply in public where possible.
A public conversation will enhance the position of you both. Keep it intelligent and generous. Everybody likes positive comments and usually responds the same way.
Be helpful whenever you can
Positive support and help in a friendly way will always reflect well. Embed any criticism as positive encouragement.