Having breakfast before I have to deal with people helps. I eat before I leave the house so it has taken effect by the time I get to work.

It should go without saying that going to bed at a reasonable time to be sufficiently rested is important too.

Work at work, don't work when you're not at work. Especially of you're hourly! Your employer doesn't own you, and if you're hourly and you're off the clock they're not even paying you.

If you're salaried they still don't own you but you can expect to put in some not-directly-compensated hours to help get projects finished or deadlines met. Don't ever let them guilt you into missing important things in your personal life though. Someone you know dies, go to the funeral. Your kid has a recital or game, be there. Friend or family in town, take time off to spend it with them. Loved one in the hospital, be there. Take a vacation, so you can stand to put up with the crappy parts of your job.

If you're done for the day and have a great idea for something at work, write it down and plan to deal with it at work. Don't obsess over it on your own time. Have a live outside of work because someday you'll want to retire but if all you do is work, you won't know what to do or who you are without it.