If someone has tons of hit points, he could always say "I can take the d6 every round, I'll just ignore the fire." This is, quite frankly, a complete violation of the idea that we need to keep metagame thinking out of the game. So, maybe the person who is on fire needs to make a Fort save every round or take Constitution damage. After all, being burned isn't good for your health.
Even so, of course, a fighter with a great Fort save could say something similar. So, make the save such that a successful Fort save means half damage. This isn't a Ref save for half damage (else the rogues could say what the fighters had been saying).
Make the damage enough to make it worthwhile to put out the flames, and then have things get worse each round.
So, something like this.
Round 1: Set on fire
Round 2: d6 fire damage, if victim is trying to put out flames, no Fort save needed, if victim ignores the fire, then Fort save DC 15 for 2d4 Con damage (1d4 if successful)
Round 3: d6 fire damage, if victim is trying to put out flames, no Fort save needed, if victime ignores the fire, then Fort save DC 17 for 2d6 Con damage (1d6 if successful)
Each round thereafter, increase the DC by 2 more, and increase the die type of the damage. It won't take long before even the hardiest metagamer decides to stay in character and put out the flames.
Dave
Even so, of course, a fighter with a great Fort save could say something similar. So, make the save such that a successful Fort save means half damage. This isn't a Ref save for half damage (else the rogues could say what the fighters had been saying).
Make the damage enough to make it worthwhile to put out the flames, and then have things get worse each round.
So, something like this.
Round 1: Set on fire
Round 2: d6 fire damage, if victim is trying to put out flames, no Fort save needed, if victim ignores the fire, then Fort save DC 15 for 2d4 Con damage (1d4 if successful)
Round 3: d6 fire damage, if victim is trying to put out flames, no Fort save needed, if victime ignores the fire, then Fort save DC 17 for 2d6 Con damage (1d6 if successful)
Each round thereafter, increase the DC by 2 more, and increase the die type of the damage. It won't take long before even the hardiest metagamer decides to stay in character and put out the flames.
Dave