Go to your profile. Under the heading Modify Profile, click on Notifications. Check the top two boxes that say, "Turn notification on when you post or reply to a topic" and "When sending notification of a reply to a topic, send the post in the email (but please don't reply to these emails.)"
Use the drop-down box to set the frequency of notification on the line that says, "For topics and boards I've requested notification on, notify me: Instantly - but only for the first unread reply; Daily; Weekly"
Then click on "Save settings."