I just thought of something to add.
You can manually add a subscriptions to a thread as well. Just open it up and click on the little 'Thread Tools' menu in the upper left corner.
Using this, you can subscribe to old threads you've posted in before you changed your settings, or subscribe to new ones without having to make a little 'subscribed' post
And to find your old posts, just click on your user name beside any of your posts and select 'find more posts by user' and they'll be shown for you.