


The fastest way to a slimmer, more fitting shirt is a back dart stitch.  You can see the 2 crescent shaped stitch lines on either side of the back of the shirt. These are designed to draw the outer waist of the shirt in, therefore creating a more fitted pattern, and therefore a more slim looking figure.
Take your dress shirts to a local alteration shop and add back darts.  Its the easiest way to increase fit and style. These Italians give a good example.
Pictures via 
thesartorialist and 
biasedcutblog
This is a good comparison picture. Left = Bad. Right = Good.
Try with slim fit pants for a complete look.