The best way we've found is to do stuff but leave the results open... Instead of "he disemboweled her" we prefer to use "he swung the knife at her stomach, intending to spill her bowels".

The former leaves the other character no escape routes, while the latter can be avoided. If someone gives us such an opening, we probably would at minimum take some damage. Of course, if the situation calls for it, we could easily let the full attack hit.

Sometimes, going out on a limb will come back and bite you, other times you'll end up having done something truly epic. If you're unsure about what to let through, you could always let chance decide by rolling dice.

No normal character should be invulnerable. It might be annoying as mosquitoes buzzing around your head if the character you spent hours or even days writing ends up dead (or maimed), but in our opinion, if it makes for a good story, it's worth it. Stories ruined because you dare not let your character take a punch when it is unavoidable without breaking plausibility cannot be recovered nearly as easily as you can create a new character.