I'm growing my latest Twitter account and it definitely seems a little bit harder than it was a few years ago. To answer your questions, my follow/unfollow strategy is like this: I follow about 60 ppl over the course of a week. Then I wait 5 to 7 days, during that time I still tweet, comment, RT etc, and I still follow back anyone who follows me, but I don't start following anyone else. After the 5 to 7 days are up, I go through and unfollow anyone who didn't follow me back. This might be about 30 ppl and takes me about 5 minutes.
I keep a sticky note on my computer about which username was the most recent of the old batch of people who followed me (so I don't have to go thru the entire list of all of the ppl I follow, just the ones at the top of the list since the last time). After doing the unfollowing of those who didn't follow me back, I update the sticky note and put the most recent follower's username on my sticky note so I know that's the end of the list for next time.
I've never had a problem with Twitter having issues with me unfollowing 30 ppl at once. In part, this is probably because I'm waiting 5 to 7 days after the last follow I do before I do any unfollowing. I could scale up my method but I haven't tried that because a) I don't really care that much about Twitter, it's not a central part of my blogging strategy so I don't want to spend too much time on it and b) I am a bit picky about who I want to follow because I really don't want political rants appearing in my timeline. Therefore, it takes me awhile to find 60 people I want to follow, since I like to make sure my timeline isn't full of junk. So I haven't figured out how long is the minimum timeframe that Twitter is happy with for unfollowing, the 5 to 7 days just works naturally for me.
