You're in the unenviable position of trying to draw the eye in two, usually opposite, directions, in (slimmer) and down (shorter), so you'll likely need to play around a bit with patterns. While things like large patterns will make you look shorter, they'll also broaden you a bit too. The advice here seems pretty good and I'd add using medium width stripe shirts along with things that break that vertical pattern like pocket squares, flap pockets or cuffs (as has been suggested) will help. The cut of a jacket can go a long way to slimming you too. If there's a store that caters to big guys in your city, try there. They'd have lots of experience with making big folks feel like they're less on display. e.g. http://www.mrbigandtall.ca
and http://www.askandyaboutclothes.com/H...Body Types.htm
As an aside, when people ask questions like this, there's often a lot of responses like "go to the gym". I know lots of big guys who work our regularly and are still big guys and look out of shape. That's just their physiognomy and for some folks, it doesn't change much regardless of their fitness level. I'm sure the advice is well intentioned, but it's not always as simple as gym=slim.