Don't the OSX devtools have a GUI for doing this? Can't remember what its called, I don't have XCode installed these days, but I remember it being pretty nifty.
EDIT: its called the Network Link Conditioner and apparently you install it from within xcode now.
EDIT2: I have just learned that ipfw is gone in yosemite? How distressing. Did they replace it with something?
I think pfctl is the suggested replacement for ipfw, which was deprecated in Mavericks. I wrote the Comcast tool someone linked in this thread. Haven't had a chance to dive through the pfctl man pages to add support for Yosemite yet unfortunately.
EDIT: its called the Network Link Conditioner and apparently you install it from within xcode now.
EDIT2: I have just learned that ipfw is gone in yosemite? How distressing. Did they replace it with something?