How do I execute well-timed taunt kills?

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.

One particular taunt kill I got, I saw a Soldier coming up the path. 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.

If you like to use the scout, and you have the sandman and the bonk energy drink, you can always drink your bonk and then taunt with your sandman to be sure you won't get killed.