This, thankfully, is not currently a problem with any groups I'm in but it is something I, and other GMs I know, have had problems with in the past. It is a simple piece of advice, and so it is a good thing for this week where everyone is distracted with the holidays and I want to keep posts brief.
As the GM you need to remember that you too are a player. You are entitled to have as much fun as anyone else at the table! Do not forget that. If you, as the GM, are not having fun then the game is going to suffer, perhaps even more so than if any other player. A GM not having fun is a GM who isn't happy, and that means that the players will suffer.
Why? Because when you, as the GM, aren't having fun you're less open to the players and what is going on. You're more likely to get annoyed at frivolty, or to be combative against random player ideas. On the other hand, if you are having fun, you are more likely to ride the energy and let things happen that may not follow rules but are a lot of fun.
In the most extreme situations, a GM not having fun can flat out kill a game. So remember, have fun as a GM and if you aren't having fun, talk to your players and see how you can make things better for yourself.