A number of NetJets Pilots use an app (name escapes me right now, I'm fatigued
) They input their show times and flight legs and it alerts them about possible fatigue. Other pilots carry around the FAR 117 rest rules and abide by those as much as possible. If they get an early start and exceed the max duty hours per FAR 117 they'll fatigue.
It is good to have something that'll alert you. I've flown "one more leg" then kicked myself because the back of my eyelids starting feeling like sandpaper.