Start out gently with a sincere reference to what the audience has just seen. Cut most of your early speaking humor and get to your subject to ease the audience's transition to your more lighthearted topic. One time I had a ventriloquist introduce me at an early morning meeting to wake up everyone and get them in fun. You could pass out fun snacks to the audience or put balloons on their chairs. Public announcements and agendas can be decorated with cartoon characters.
Funny props are great for putting people in fun.
Do anything you can to be sure your audience knows that it's OK to laugh.