Look on the top of the forums page for User CP (stands for Control Panel). Click on that, then scroll down the list on the left of the page to Edit Potions. About halfway down that page is the Default Thread Subscription Mode. The drop down menu is what you're looking for.
Member of Book Worms Clique and proud member of INCREDIBLE INVISIBLES.
Mr Wigglebottom 1999-2010. Once in a lifetime cat. Much loved, much missed.