You must have the strategy to do it. Which mean you need to do it correctly in order to win the contest. Personally, I have got my very own strategy which is simple as 123. Everyone can do it.
First you need to spot the contest and do yourself a favour by taking part in the contest. After you have done the requirement all you have to do is start commenting.
Always check back on the comments that you did. Who knows you might have feedback or response from them. It's important that you always keep track on the comment contest. You do not want to lose a chance in commenting. Most comment contest will prefer the participants to comment without spam comments.
Everyone has mistake in the beginning. Well, I still have mistake and I am trying to correct them on the way.