Two points that I see most important when it comes to commenting on others blogs' are: 1. make it easy--having to sign in each time to comment, makes me not want to comment. 2. comment on the comments people leave you, interact with those reading your blog, acknowledge them, thank them etc..for leaving a comment.
I regularly try to comment on a couple of blogs that I keep up with. The two blogs I follow are of friends from church. One of the ladies has/is going through cancer and she keeps us updated on her blog to her progress. The other blog belongs to a friend who does public speaking and her blogs are just so real to life, and seem to minister to me quite often. I also like to keep up with my principals blog.