Jan 27th 2009 3:18PM The method I use, but might not work for everybody, is to set up a macro that just has "/assist name" and nothing else. Put that on a sidebar and map a button to it, in my case I use the tilde key (~). Open up your macro window with /m and type in the tank's name whenever you need to change it. Just press ~ to assist the tank after that.
I know you can set up a neat macro to set your assist target if you hit ~ while you're targetting a friendly, I've just never looked it up. This works well enough for me.