Top posters appear in a list within each game. This list get updated every 6 hours. Becoming a top poster can be challenging, but you can improve your chances by following these tips:
1. Post often--daily if possible. The algorithm favors people who post at least once per day. It doesn't hurt to post more, but you should post once per day at least.
2. Get featured and enter challenges. See how to get featured for details about getting featured. Entering challenges is another a great way to give your clips more exposure (especially if they rank high).
3. Be a good community member. Take time to like, comment, and follow other gamers. The more positive and visible you are, the more likely others are to watch and like your clips in return. Note that following or unfollowing too many users at once can hurt your top poster ranking.