To this one, make some spam comments (each one has to be different) just to make sure you get your comments in. Then, whenever the video is close to or reaches the 500 comment mark (which is when it locks down), delete some of your spam comments and then make sure YOU are the 500th comment. This means that YOUR link will be up for MONTHS, and every single person who watches that video will likely see it. As the channel grows, so does your reach