If it's a stalemate and you're just spamming, heal the flank when they come for heals/buffs (ideally, when they've taken dmg on the flank they'll back up and wait a bit before coming to you so they can have crit heals) and then resume healing the combo. Make sure no one on the combo is being overly aggressive/peeking when you're buffing the flank.
If a player is on the front lines and getting shot, heal him. If someone is just standing behind and not taking any damage and bitching about not getting healed, tell him to stop bitching and wait a few secs.
If your entire team has shitty health you all need to step back, give up some ground and heal up so you don't get wiped
EDIT:
concor-Keep all flashes equal between all. Follow your designated combo and give heals to those who need it.
This is bad advice. You should never flash your entire team equally, that just wastes your uber and makes it super short. If you're just going for an uber trade and you want to force the other team, you should focus on a solo uber. Only flash people that need to be flashed and only make the flashes as long as absolutely necessary (while they're being actively shot at - you need to use the uber to block dmg). The idea is to keep your uber active as long as possible so it lasts longer than the other team's uber - that's why, while ubered, your team has to try and shoot the non ubered enemy players and force their med to flash them and waste his uber duration so you guys get a better (as in longer lasting) uber.
EDIT#2:
Some other useful crap - healing isn't just your job. It's the entire team's job to rotate the heals, and not just all come to you at the same time on 1 hp with no crit heals. Tell some of them to get a pack. Also tell them to keep track of when they've last taken dmg so they know if they have crit heals or not. That way they'll know the best time to approach you and ask for heals