Since no one is replying and I'm actually curious of this myself. I'll point you towards this video.
https://www.youtube.com/watch?v=gkh9bfylLkI
He has a full tutorial on how he made it, but basically it's a turret that faces you all the time if you're in range by using a trigger_multiple to its left and right that turn it left or right when you're in each of them.
Following the player directly though.. That might be a problem. I'm assuming you want it to lag behind at times - slowly follow the player. If you're just talking about having a prop_dynamic on you at all times you can set it's parent to !activator off of a trigger_teleporter, and put the destination of the teleporter directly overtop of the prop (without collisions). Or however you choose to do that...
But, following the player, Booj used a logic_measure_movement on his 'Chairman: The Derby' to make something that couldn't be parented - be parented. You can increase the scale thing on how it moves relative to its connected entity. So maybe you can set that to like 0.5 if the entity you want to follow the player is outside a certain area. SetTargetScale is also an input variable, so that works well. So using that it could make the follow entity move closer to the player if the player walks away until a certain distance. But then if the player walked agaisnt it then it would just move farther away really fast aswell.
I donno, you can probably find something mixed in with all that using weird triggers and shit to make it work right.
Here's the logic_measure_movement entity:
https://developer.valvesoftware.com/wiki/Logic_measure_movement
and here's Booj's chairman thing incase you were interested in that too:
http://forums.tf2maps.net/showthread.php?t=13412