I generally use a combination of three methods:
1. Set up target macros for the two or three main tanks, if you're not in the tank party;
2. Use <tab> and first three name letters; and/or
3. Just type the names into a target command.
There is no ideal solution - but each party in an alliance should have someone with healing capability in it, and those people should be primarily responsible for healing people in their party. I also have a few practices I try to follow:
First, I always talk to the alliance leader and make sure what my role is. If it is to keep the tanks up and to hell with everyone else, so be it - but then I also make sure everyone else knows that too, so those with /whm can keep others cured.
Second, in the absence of non/pt-tank curing duty, my pt comes first. After that, if I can keep the healers in the other parties healthy I'm gonna do that.
Third, I try to figure out which non-tank is consistently getting a lot of damage (we call them "MP sinks") and, if I have a free macro key available, I may create a target macro for them.
I've been the only whm in alliances before. It's not fun, unless there are good non-whm healers in the other parties or unless most members are good at hate/damage control. Even then, it's not necessarily easy. I remember one time in particular, going for Uggly Pendants, where I was the only whm. I spent my time raising people and desparately trying to get at least 200 mp so I could keep people going. Decidedly that was one of my least fun whm times.
All the best,
Fire