When you read a hub that you like and believe you have something to contribute besides just "Good Hub", then leave a comment.
Follow others and they may also follow you. Personally I mostly follow either the top hubbers who I feel that I can learn from, or whom publish hubs in topics I like.
Occasionally ask or answer questions.
Leave comments in the Forums.
Leave comments on the official HP Blog.
#1 Would be writing Hubs that others will find interesting. If a person likes your hubs they are likely to follow you.
Basically every place on HP that you can get your name seen, you are increasing your chances of gaining new followers, getting more users to read your hubs and making some money possibly.