I've always liked how Deadlands handles Blessed (cleric equivalent) characters. Blessed characters are required to follow the tenants of their religion in order to retain their powers. Deadlands is an alternate wild west setting, so most of the religious beliefs are going to be real life. So a Baptist preacher couldn't drink alcohol, a Muslim Imam has to answer the call to prayer five times a day, etc., etc. But there are different levels of infractions, if an imam decides he just doesn't want to pray in the morning then it's a minor infraction and he'll be penalized on his Faith rolls. More serious infractions such as murder or renouncing God would result in stiffer penalties including loss of access to Miracles (spells). (And for the love of Pete! DM's shouldn't use these religious rules to place player characters in got'cha situations.)
In both Deadlands and D&D, these should be tools to enhance the experience for the players. Playing a cleric, a representative of a god, should carry with it some responsibilities including living according to the values their divine source says they should. Otherwise what's the point? Wizards, clerics, bards, and warlocks are all the same. Boring.