#1 Create and Share Content You Find Interesting
It should be obvious that, before using any kind of special tactics, you need to create and/or share content that you find interesting. You need to create value. Start there.
#2 Cross Your Fingers
And wait. Seriously. Don't do anything else. Don't "follow back" just for the sake of it. Definitely don't ask people to follow you back. Don't mind your following / followers ratio. Post whenever you feel like it. Use hashtags, or don't.
I see people with hundreds of thousands of followers, following hundreds of thousands of people. I can only assume as part of a follow back scheme. Why? Like, really? You're keeping up with a stream of tweets from over 100k people? I can't imagine you have time for much else.
If everybody did that we'd all have millions of followers, and no one actually reading anything we say.
A handful of people that are genuinely interested in what you have to say and share are worth more than an artificially inflated vanity number. Nobody that matters cares about your number of followers. Don't waste your time.