This is a mutual help/ series growing thread, but it's not 'Like-for-Like'. (2nd take)
Like only when you want to. No need to return favors.
Don't forget to show your series total likes, your goal, and also your series' link.
Put out a realistic goal number. Once your series reaches that goal, you can reply for thanks! 
edit: When helping out other series, TRY not let others know your help (to reduce the 'like-to-like' tendency) and just simply like or leave it for your own accord. Maybe you find something series you really like and be a subscriber of its creator by the way!!