There seems to be one overriding rule: keep it short and sweet (less than 4 minutes), and two broad schools of thought:

[1] wrap little chunks of programmes into a 'compilation' which will demonstrate some degree of editing as well as camerawork and general style, but makes for a longer reel.

And you're somewhat at the mercy of the clients' requirements if you shoot Corporates. You actually have to identify with the subject matter.

[2] pick some of your favourite (how do we define 'best'?) shots and sequences, some 'before' and 'after' (e.g. chromakey or grading) and some motion graphic stuff and beat until stiff with a high energy sound track.

But it's really a peacock tail. Looks great, not that you'd want a finished video that actually looks like that.

How are you approaching your reels?