I use a TAB (or shift+TAB) to cycle through mobs (shift+TAB for backwards cycling) next to clicking the nameplates. There's a few very handy addons for managing the nameplates and I recommend you check them out if you haven't allready. Clicking the nameplates is by far the most exact targetting way I know off. When I want to spread out threat evenly I tend to use TAB though.
I know of no macro for switching targets based on priorities. Of course you can make "/target" macro's for specific fights if you think it might help you. Try out a couple of methods and see what you like.
My lack of proper signature actually is a proud minimalistic stand