There is a key binding to set a focus target. For example, I target my companion, press my keybinding to set a focus target (ALT+F) and the focus target frame appears. When I am in combat and want to heal my companion I press the focus target modifier key binding (I bound my key to F, but it was by default either the Home or the End key.) While holding down the focus modifier I then cast a heal and the heal goes to my companion (or whoever is my focus target) without changing my target. I can then target myself by pressing the self target modifier key. I don't remember the exact key bindings or terminology but I hope this is enough information to help you do some experimenting. I have to say that there are a lot of times when the focus target seems to disappear for no reason, and it seems that focus disappears when zoning or under any number of unknown conditions, so keep an eye out for that and then re-set your focus target. It is buggy, but for a healer with a tanking companion it is very useful.