IMO The biggest thing to getting a RL to let u MT is to overcompensate. When I say overcompensate I mean to make it seem like you are the best of the best available to them. This sorta feels like it goes without saying but it almost feels like a fulltime job sometimes. When the RL thinks MT he has to think YOU over the other tanks in your guild. Even volunteering to OT or switch specs can take emphasis off you as a tank later on down the road.
The reason I say you have to overcompensate is because almost always for any raid spot out there other people are going to want it just as much as you are even in a casual guild. Your other guildies whether cut-throat or casual buddies, may not specifically want your spot but they still want a spot and if the RL thinks they would be better at it then you then you have lost most of the battle already.
And lastly I'd say a mixture of discretion and boldness to know when is pushing too much and to be able to keep your other guildies confident in your abilities as a MT.