(Sorry to be responding so late, didn't see this thread before.)
One method I've experimented with, that might work if your players aren't too "hack & slash", is to treat any attempt to resurrect someone as a role-play challenge. Rather than just chanting over a corpse or whatever, the character who's performing the resurrection rite would psychically visit the Land of the Dead, where they would serve as an advocate for the dead person in a confrontation with my campaign-setting's embodiment of Death (the Hound). The challenge would be for the priest's player and the player of the dead character to come up with a reason why the deceased's death wasn't an appropriate closure to their particular life - not pragmatic reasons like "the adventure's not over" or "the party needs them", but something personal and consistent with their personality - by which to justify their being granted another chance to get their death "right". In-character, it's an appeal to the Hound's sense of aesthetics, that people should die in keeping with their principles or lack thereof; out-of-character, it's a reward to players who've gone deeply enough into their characters' personalities to come up with a good argument.
It's not exactly making resurrection any harder - the Hound is a reasonable sort, if spooky, and I never actually refused anyone's restoration so long as they at least made an effort to argue their case - but it'll make the players take a character's death more seriously if they know they'll have to essentially win a debate in order to get their PC back.