-- Having a job where you don't go in every day thinking that today is when they will find a reason to fire me or get laid off.
-- Having a bit of surplus personal money (after retirement accounts are paid into, and other planned savings) is really good. It doesn't take much, and you don't really need extra income, just make sure your lifestyle matches slightly below your income.
-- Knowing you can survive for a while without a job, if needed.
To accomplish the above (without having to significantly increasing your income), make sure to avoid the trap of monthly payments as much as you can. Sure you can afford $800 a month on a car payment, but your emergency fund stretches much further if you buy a car with cash up front (or going no more than 3 years on a loan).
-- Take time for yourself. Don't leave vacation days unused. Take advantage of flex time without using vacation if you can (cutting out for a couple hours for a Dr appointment, etc). Take a couple Wednesdays off per month during the summer (5 weeks vacation gets you a 4-day work week every other week for the year, or take minimum time off in the winter, take a few 3-4 day weekends, periodically take a day off that you don't have "must-do" activities scheduled so you can do personal projects such as woodworking or gardening, etc).
This is what has kept me happy, and will continue to do so even if I have to work till I'm 65.