Some tips I've found useful (some that have already been mentioned):
1. (mentioned above) Create lists. And then interact with the people on the list. If there are agents or reviewers or book store owners you like, reply to their posts, even a simple click on the heart (now a "like" used to be a "favourite"), notifies them of your existence.
2. Authors are great fun to follow, and I highly recommend it simply for expanding your community and support system. That being said, for self promotion purposes they aren't that helpful (unless a really famous one retweets you) so make sure you also follow librarians and book store peeps. As well as bloggers and reviewers.
3. Hashtags are a great way of reaching people not following you. Of entering conversations that are larger than just your Twitter world.
4. Do not, I repeat, do not spam about your work all the time. Seriously. Don't do that. Be yourself. Post funny observations. Retweet other people. Support others. No one trusts an author who only talks about writing and their books.
5. It's wise to attempt to get your follower/following balance so that your followers well exceed the number of people you are following. If it's one to one it looks like you are just trying to get followers by following every single person. That you are indiscriminate. I mean look at someone like Neil_Gaiman: he has 2.6 million followers but only follows 868 people. So that's a bit extreme, but the point is, people will judge based on the ratio. You said above you are following way more people than you have following you. See what you can do to even that out and then bring up your followers. Check and see if you really do need to follow all the people you are following. I bet there are some you could unfollow.
6. Manage the number of people you are following by using a site like this:
http://untweeps.com/ This tells you how often the people you follow are actually using Twitter and how large their outreach is. I started ages ago on Twitter and so a lot of the accounts I followed have since been abandoned by their owners. It's a really fast easy way to check up once and while.
7. You asked how one keeps up with everything posted. Well one doesn't. You simply cannot go back through your feed and make sure you've read everything. That's not how Twitter works. You see what you see in the moment you are on Twitter. You miss a lot of stuff. That's the nature of the beast. Don't attempt to comment on every single tweet made by others or read every single tweet. That way madness lies.
But above and beyond everything remember that Twitter is a community. You are there to promote but you are also there to interact, get to know people, have fun. Share tweets by others, support others, celebrate others. And yes, you can absolutely post about your stuff too once in a while, but if you make your focus more on community building, it'll serve you better in the long run. And you also won't feel quite so icky about the self promotion aspect.
). I've learned so much in the almost decade or so I've been doing this author thing (like seriously 2017 it'll have been a DECADE since I published my first book. Blows. My. Mind.) and I love to share everything I've learned whenever possible. And especially since my learning curve with Twitter was very slow, took me a long time to figure stuff out. The first few months I didn't even know people @ replied to people on Twitter. I thought that you just wrote a tweet and others read it and that's all, lol. So anything I can do to speed up that process for others, well, I will do! 
