First you will need an addon like Atlasloot.
When you are at low health you can add some rare item in the Battleground Channel to DC yourself.
Use items like "Warglaive of Azzinoth" (Both if they have dropped on your server) to Disconnect yourself. (Some servers haven´t dropped those items yet. That´s the reason you get DCed)
Now QUICKLY loggin again and go on your char and then you will have 100% health and mana !
EDIT: You don´t need to use the Warglaive. Explore atlastloot to find rare items on your server and then link them in Battlegrounds. Of instance I sometimes also use "Murloc Costume"