Commenting seems to be the most lucrative way of earning steem on steemit. If you think about it, curation rewards are very low and unless you have a big account you won't make much by just curating content. On the other side, commenting can be very lucrative. I've seen comments with 1000 USD rewards.
How to make money by commenting on posts
If you like a post, you can of course up vote it, but i would suggest you to write a comment with your up vote. Try to respond to the original post with some helpful information. In most cases if the OP of the post is in a good mood he will up vote your comment and you will most likely earn some money. But don't just say things like "Keep up the good work" or "Very nice article", those comments are flattering but useless and you won't get any up vote.
Get noticed by the whales in the comments
The easiest way to get noticed by the whales is not by posting a full article, is by commenting on articles that are already popular. Whales are visiting those articles, they are up voting those articles and most importantly, they are reading the comments and up voting the best comments.
Whales want interaction, they want to build a community
Whales are trying to build a full scale community on steemit and the only way to do so is to create activity. They want people to interact with eacher other, they want articles to get as many good comments as possible. How do i know that ? If you read the github conversations between the admins you will notice that they didn't want to penalize heavy posting of comments because they want people to interact as much as possible. They did penalize heavy posting of articles but not comments.
Examples of comments who got well rewarded
End word
I wish you all a happy commenting and good rewards ! And don't forget to add value in each one of your comments.