Here's my system if I was design a system. A cleric has several different types of magics available to her--blessings, litanies, rebukes, and rituals.
Blessings are the lowest and easily most available kind of magic a cleric has. They are simple prayers to invoke a blessing upon the cleric or her allies. Present a holy symbol and utter a blessing. This is curative magic, buff spells, and similar protection magic. Bless, protection from evil, cure light wounds are examples.
Litanies are more powerful prayers. They are quoted directly from the order's scriptures or their "bible". The litany must be read even if the cleric can quote it from memory. This provides attack spells. Flame strike and blade barrier are examples that I'm thinking of.
Rebukes are curses and damnation litanies. Turning undead, smiting, and similar spells that require an attack are examples. The cleric's holy symbol and weapon in hand is required for this.
Rituals are done in special chambers, holy sites, require expensive sacrificies, long meditation, and reading of litanies from saints or devoted scholars that is considered sacrosanct within the Church's order. They are long and costly and invoke the most powerful magic the god can provide to his followers. Raise dead, resurrection, miracle, etc are examples.
A cleric starts out with a number of blessings. A cleric can invoke as many as she wants or has a large number within any given day. As the cleric gains levels, this number increases and they gain access to litanies, rebukes and ultimately, rituals. The limits placed on a cleric is not so much as "fire and forget" but in that their god is only able to infuse so much of its power in such a weak vessel. If he was to give more, the cleric would explode or burn from the godly power.