In many cases, the person with low health has low health because a mob is stabbing them in the face, thus frequently targeting them. So try a macro that casts heal on your target if it's friendly, on your target's target if the target is hostile, and optionally on yourself if you're not targeting anything.

Obviously doesn't apply to AoE situations, is much weaker in PVP situations, and doesn't help if several characters simultaneously have aggro on separate mobs. But it's handy in PVE for the other 95% of the time.