Let's first look at WHY you would want to give quality comments to others.
1. If you give quality comments, you are more likely to receive quality comments back from others.
2. Every comment you leave is essentially a backlink to YOUR website. This is great for you too.
So how do we give quality comments that work for you and the other website owner?
1. Comment on the CONTENT of the post and make your comment RELEVANT to what is being said.
2. Ask a question so you give the website owner an opportunity to reply back to you, possibly even to link to other content on their website. There might also be a really great point the writer didn't think of, this gives them an opportunity to expand on their content.
3. Write a long-ish and thought provoking comment that adds good value to the website.
4. Add in YOUR keywords too if you can, IF you can make it look natural and if your keywords are relevant to their content only. This can help to improve YOUR rankings.
5. Write properly, as if you were writing a post on your website. Watch your spelling and grammar.
Let's look next at what NOT to do when giving comments
Cheers!