If there is a thread that you want to keep track of, just click the Thread Tools menu at the top of the first post.

Choose 'Subscribe to Thread'.

You have the option of just subscribing to it, or subscribing with instant, daily or weekly email notification.

The daily and weekly emails show a 'digest' of all posts since you subscribed.

You can set this to happen by default when you post a new thread or a reply to a thread by going into your UserCP and choosing it in your Options menu, or just choose it at the bottom of the page when posting.

All of your Subscribed threads are displayed on your UserCP main page by default, and your last 5 posts and threads started are displayed beneath them.