Another way of doing it is using the focus modifier key option, and simply holding that button down while using your normal buttons. It has other advantages too, such as being able to quickly throw a wrath or starfire on your focus as well. Imagine that you are doing Valiona & Theralion, and have the airborne dragon as your focus. If you don't have another source of the spell damage debuff, you probably want to throw the occasional wrath or starfire on your focus to keep up Earth and Moon.
I've set it up with the ALT key, but you can choose between several different buttons.
2: insect swarm
ALT+2: insect swarm on focus
3: moonfire/sunfire
ALT+3: moonfire/sunfire on focus
R: wrath
ALT+R: wrath on focus.
Here's a picture of the relevant part of my ui:
http://img10.imageshack.us/img10/7517/timers2.jpg
The big frame is my target, and the small one my focus target. The timers (using NeedToKnow) above the big one is for my target, and the ones over my focus frame is obviously tracking my focus.