I find that follow-unfollow is the best method. It can be very effective if done correctly.
I would gladly pay for shoutouts, but I haven't been able to find sellers. Most don't even reply to my messages.
Joining a tweetdeck could be effective. They scare me, because who knows what some fool in the deck might tweet on my account? I've never been part of one, so I can't say for sure what it's like.
If you get enough retweets, there can be organic growth on your account, but this seems to be mild. Perhaps when your account is already huge, this type of growth is more significant.
I believe that paying for actual Twitter ads would be prohibitively expensive.