Doing a little preparation before your class visit can make a big difference in what both you and your colleague get out of it.  The following are some tips to help you plan for a more meaningful observation:

1.  Be clear about your goals for the visit:
Be upfront with your colleague about what you're hoping to take away from the visit.  Are you looking for feedback on a particular aspect of your teaching?  Are you looking for new ideas to implement in the classroom?  Are you hoping to see a particular pedagogy in action?  Communicate your goals before the visit so your colleague knows what you're expecting.

2.  Put the class into context:
Observing a class "cold" can be tough -- it's hard to make sense of what's happening when you don't know the larger goals for the course.  At the minimum, share a copy of the syllabus, so your colleague understands how the class he's observing fits into the broader scope of the course.  Sharing relevant readings or other assignments can also help your colleague get a sense of the context for your class.

3.  Have a plan:
Thinking ahead about what you want to pay attention to during the observation -- and how you'll take notes -- can make it easier to focus on what's important.  To help organize your observation, you might choose to focus on a particular "lens," to keep "double-entry notes," or to create a "discussion map."

You might also check out these resources developed at other institutions: