Most Steemians will upvote comments that enhance their post. So, this is a great way to earn SP.
There is one tiny catch: New users on SteemIt have so little steem power that their upvote is worth less than the rounding error.
When I started SteemIt. I left about 100 comments on small accounts. I was really disappointed to find that I did not receive SP for any of my comments.
I guess the first solution to the problem would be to check the SP and upvote record of an account before leaving a comment.
Personally, I don't like like doing this.
Because I like interfacing with small accounts, I started using the @dustsweeper bot.
If an author with limited SP upvotes my comment, then the bot will come in and add enough voting strength so that the vote total comes in over $0.02.
Leaving comments is a great way to earn SP. Unfortunately if your goal is to earn SP; you should only leave comments on accounts that have at least 150 SP; so that the value of the upvote you receive is over the rounding error.