First, make sure that your account preferences is set to automatically subscribe to posts. You can do this by going to Forum (top light blue bar) then Forum Actions (dark blue bar) then General Settings.

Scroll down to your Default Thread Subscription Mode and choose the setting you prefer.

You will also be given the option to subscribe or not when writing a post. When replying to a post, if you select Go Advanced, below the text box will be a list of additional options. The default you selected in General Settings should be preselected, but here, you can tailor your choice to the specific thread or post.