Big Channels
Replying helps a ton as long as you are either the first or second person that has replied to them. It can be hard, because you need to say something that intrigues the audience and the YouTuber. If you spam your channel right away the YouTuber may end up blocking you. Also, you need to be extremely fast. First replies are often "ily", "follow me", etc. These are short and take no time at all.
I've done it before. I spend a few days doing nothing but Twitter. You get good at coming up with clever things to say in an extremely short period of time. It did help me but can be extremely frustrating. 1 second to late and you will be pushed down 8 replies that say "ilysm".
Small Channels
Become friends with them and have public conversations about something with each other. It can really be about anything. This way their audience will know that you might be somebody that they would enjoy watching since their are "agreeing" with someone they love.