I wear dockers quite a bit and know your problem. I recently bought a holster called the "Pager Pal". It is an excellent design, unfortunatley, Im not tall and thin like you, rather about 5'10" 235lbs. This holster slips into the front of your pants and completely conceals your gun with nothing showing except your pager. To draw you simply grab the pager and pull the whole holster out to reviel the gun. For me, this system does not work the best because of my......larger than average waistline, however for a thin man, it is an excellent design. You can usually find them at gunshows.
I have resigned myself to carrying in a ankle holster when wearing dockers. I carry a titanium 342 and find it hides very well on the ankle. For a larger gun, your gonna have a tough time concealing it in just dockers and a sport shirt. Maybe a belly band or thunderwear, but Ive never been real happy with those concepts.... again, not designed for us big boned folks. Good luck, and if you have troubles, maybe you should invest in a jframe 38 or a smaller gun like a guardian or secamp? Hope this helps