This is the strategy that I've used and it's been pretty effective. It takes constant optimization but if you stay consistent with the process you'll quickly build your follower base. Also keep in mind that a lot of the followers you gain will not be quality followers so this strategy is really just for building numbers and not building your interactions.
What I do is go find an account with lots of followers that tweets similar topics or information that you do. Open up their followers and begin to add anyone who follows them and has tweeted a message in the last couple days. This tells you that first off the person is active so they should see your follow...which raises your chances of them following back. Second, since they recently followed the person's account you are going through, there's a better chance they will be interested in what you tweet about as well.
I usually do this process above every Sunday or Monday and I'll follow anywhere between 150-200 new people. Wait till Friday (so 5 days) and then use this Twitter API http://manageflitter.com/
It lets you see who is not following you back, who you are following that is inactive, and several more. You can then do a quick bulk unfollow for those.
Essentially it's a add, rinse, repeat type of process but it works pretty well.
As for creating good followers who you want to interact with...well for that you'll have to actually interact with them to get them to follow you. I usually start off by creating lists before I start the mass follow process so that I can easily organize my valuable followers from the followers who I don't care about and just want for stats reasons.