Go to "Settings" at the top of the forum page where you sign in, find "My Settings" and click on "General Settings" and find "Messaging and Notification". Under that there's "Default Thread Subscription Mode" where you can choose the kind of email notification you want.
If you want specific threads only, to email you for new posts, go to the Thread Tools link you'll see across the to of the first post in a thread. In there is a label "Subscribe to Thread". There you can also specify how you want to be notified. I'm pretty sure the Settings functions in your Control Panel will affect all threads the same. So every one would either notify you, or not notify you, if you happened to post in it. Much less clutter if you subscribe only to specific threads.
