Poker stamina for multiple table players

[Uglyowl]

How do you handle the mental fatigue that sets in. For me it seems to go down hill after 1.25 hours of playing many tables.

Ideas I have batted around is reducing the number as the time wears on. Taking a half hour break in between.

Does your stamina improve as time goes on?

Seems like a silly question, but I am serious. Every mistake adds up.