Conversations are key
But you should also go further whenever you can. And by that I mean don't just retweet someone a few times, or say "good point" in a mention. Actually try to start conversations that go on for a while.
You've got to sense when this is appropriate. Just as you can't force someone to chat with you in a real social situation such as a dinner party, it has to develop naturally on Twitter, too. So, you find that many attempts will not bear fruit.
But some will. And they'll keep recurring with the same people. You'll really stand out in their feeds as "top drawer" tweeps. And they'll often be happy to help you with retweets, etc -- especially if you do the same for them.
Influencers can help you greatly
If these people are influencers then the benefits are obvious. But even if they're not, they may well become influencers in time. And you can build your folllowings at the same rate and help each other out. It's these people who will be "the gift that keeps on giving".
This is why it's so important to have a real, genuine interest in what you are tweeting about. You should also genuinely want to discuss it with others. And make sure that the people you try and engage in this way look like they will. That's why constant use of the site is important.
It's like when you are at a social event in the real world. You don't just talk to anybody. You mingle with the people who look attractive to you, who you know already, or who have heard about from others you know and trust.
Always be sincere
If you just go into it because you've heard that you should engage with others, but you don't have any real desire to, then it's pretty clear to others.
I know from experience. Often I'll get a tweet asking how my day was or something like that. Then I'll go to their stream and see they've been saying exactly the same thing to numerous others. It just rings hollow in my opinion.
I know others might not feel this way. But I do think it's better to only mention someone when you really feel like it. That way, the odds of a quality conversation -- eventually leading to a lasting connection -- go right up.