The standing rule I had in every edition of D&D was this: If your familiar doesn't participate in combat, he won't be targeted in combat. If you don't use him to scout, he won't be the target of traps or hidden enemies. Boom, problem solved. I can understand why folks don't like this idea because in it makes their familiars feel less "real," and less involved in the campaign, but my experience was the opposite. By players not having to worry about their familiar dying from bad dice rolls, they're more willing to make them a part of the character's backstory.
|Scabbers, he rat familiar of Ron Weasley, is a great|
example of a familiar with a developed background.
Another option I had in one game, many years ago, was a player who wanted a monkey similar to "Jack the Monkey," from the Pirates of the Caribbean movie franchise. Sure, why the hell not? But, he wanted the monkey to actively participate in combat, scout, etc and still be immortal like the zombie monkey from the films. I thought long and hard about this before finally coming up with something. Sure, the monkey was effectively immortal - but because it was undead it could still be turned or potentially destroyed by a cleric. This gave the familiar durability, but the players were still cautious. It worked. I was surprised and it lead me to decide that a good method was to have familiars be effectively invulnerable, with the exception of one particular source of damage. In every edition of D&D and the retro-clone this has worked out swimmingly.
Whether it was my wife's wizard who had an eagle that firmly believed he was actually a roc under the influence of a terrible shrinking curse and his pride lead him to be easily influenced by charm and illusion spells, or the wizard/thief I once played who had a ferret who might get distracted in battle by a shiny object for several rounds and was effectively removed from the battle - familiars, like PCs, just need a little push to make them unique. It's not that they can't die, and they shouldn't be immortal - but they are part of the player character's story, and need to be respected as such.
|Respect the monkey, but don't trust him.|