One thing not mentioned.. If you are a fairly fresh 50 it's likely that you're having the problem due to better geared DPS, it's not a bad thing, just keep in mind you will have to try extra hard to maintain aggro.

Having a prepped RoH ready for the add (or after a Provoke) is your best option, if Fight or Flight is available it will help tremendously. It's a healthy habit to get in to given the many adds and tank swapping mechanics you will be dealing with from here on out.