I'd go with PirateCat's idea. I always rolled in the open, it adds to the suspense. For 'to hit' rolls as well as damage. Only rolls for checks etc. may be rolled away from sight. Every now and then, some really big bad guy rolls a couple of 20's in a row, a roll for a fireball damage turns up almost all sixes and fives, and if the PC's are too dumb to disengage when the tide is turning, sure, they'll die.
Stricly enforcing rules (I can't remember if it's official rules or simply homebrew, but they are pretty much standard in all my campaigns) of a permanent decrease in Con (-1) and a setback to the lowest amount of XP in the PREVIOUS level (i.e. the PC also loses a full level of XP), make sure that even though the party may have good access to resurrection type magic, death is a thing to be feared...