A tip I have for tank swaps is to swap on animation locks if a boss has them. Basically a animation lock is where a boss cannot move, cannot cleave and is usually powering up a move. Good examples of this include Titan ex upheaval and turn 6 devour and spit mechanic. Timing your tank swaps with these animations make the tank swaps clean and simple.

As for enmity like some people have mention, best way if your tank who is swaping with the main tank is to charge up a butchers block combo and wait to fire off the charged butchers block, same thing for paladin's rage of halone. However its not the end of the world if you can't get a charged attack, basically the idea is follow up with a enmity move right after provoking. If your the main tank know when the tank swap is coming and squelch any enmity moves your doing until the new main tank has a comfortable lead in agro. Keep in mind that all attacks do enmity so be really careful if your still close in enmity on the agro list.