My best advice is to predict where the enemy is going to go. For example, if an enemy was going down the vents in Turbine, you may want wait behind a bend and initiate your taunt kill, so that it will hit the enemy when he turns the corner.
Here is one taunt kill I got:
When I saw the Soldier coming up the path at about 7 seconds (it's a little hard to see him, he appears from under my gun), I retreated and waited behind where he was going to pop out of, and taunted.
If your goal is specifically to get a taunt kill, you may want to favor certain classes. Heavy has the fastest taunt kill, with Pyro's Hadouken slightly slower than the Heavy's.