#Simplest: Apply the morale rules to player characters, too.

For example, the old D&D 2d6 morale check. Use the morale listed in the racial entries. (Dwarf, Elf, Halfling=10 if leader alive, 8 otherwise; man 6-11). In the case of human PC's, let them pick in that 6 to 11 range.

And then just apply the rules to PC's as well as NPCs.

#Not so Simple Convince them that head-on fight-to-the-death leads to dead PC's. start with just simply outnumbering them, and/or giving them a few fights that they simply can't win. After a few TPK's, they'll start to learn.

Or they'll get peeved and quit.

It helps to give a full XP award when they decide to flee an encounter. Also, if it was the right choice, a good RP award.

Simplest:

Apply the morale rules to player characters, too.

For example, the old D&D 2d6 morale check. Use the morale listed in the racial entries. (Dwarf, Elf, Halfling=10 if leader alive, 8 otherwise; man 6-11). In the case of human PC's, let them pick in that 6 to 11 range.

And then just apply the rules to PC's as well as NPCs.

Not so Simple

Convince them that head-on fight-to-the-death leads to dead PC's. start with just simply outnumbering them, and/or giving them a few fights that they simply can't win. After a few TPK's, they'll start to learn.

Or they'll get peeved and quit.

It helps to give a full XP award when they decide to flee an encounter. Also, if it was the right choice, a good RP award.