You use castsequence macros to space out heals so that your healer is neither spamming heals nor letting the tank die. Use hard cooldowns on the healer side to space out the sequences An OHSHIT button that uses the healers big cooldown with one press and the tanks big cooldown with two (using two step macro from isboxer) is also pretty key to this strategy.

I don't have my live shaman macros before me (and waiting until cata dungeons to learn how to heal might have been a pretty scary mistake thats going to take you longer) but something like this for a priest works well

/focus tankyname
/castsequence [@focus] power word:shield, prayer of mending, heal

You put that on your 1 key (or whatever you spam for dps)

will automate a simple heal sequence that takes up about 5 of every 15 seconds. Interrupting that sequence to throw emergency heals is tricky though, that's why it is safest to use a sequence with a hard limit in terms of % of the characters time playing.