First please go to ground from Exit A of Jishuitan subway station and then walk eastwards along the road about 5 minutes. Actually, you will see the Deshengmen Tower easily once exit the metro station.
Tips: the 919 bus stops at the back of the tower, where you have to line up to board bus. Don't take the bus stop in front of the tower.

Mr Jamie, thank Y for your expand very detail, so many Chinese in Indonesia like me can not speak and understood, I wanna going to solo traveller 12 -26 Nov 2012, I really falling in love with Great China, this is second time I go back to China, this Information really useful for me... Really pround of Y... Bravo