Model it.

 
Do you have fun at work? If so, is it obvious or do you keep it to yourself? If it’s obvious, great – don’t make it a secret. If you don’t find any joy in at least some of your work, perhaps you need to look harder :)

Look for it. 

As you watch the work of your team and as you pay attention to the attitudes of your team members, notice the enjoyment and fun. Help people see what you see, and help them recognize that there is some joy in their work!

Promote it. 

When you see it, let people know it is a good thing and that you want them to continue to inject some fun and laughter into the work. Give people the space to "play a bit", and allow some fun rather than expecting “all business.”

Create it. 

Look for ways to put more fun into the work – that could mean a planned outing or event, or any of 100 things. Most important is that it is something viewed as fun by others and not mandated fun (“OK team, now we are going to have fun.”) Perhaps the best way to do this is to ask for volunteers to be on a fun committee that creates occasional and appropriate activities to inject life, enjoyment and fun into the work.