It's been like 8 months since this post was made, but for my demo reel I just wrote my own scenes.

I wrote one scene with multiple characters, one sample of poetry, and one short excerpt (with permission) of a finished - meaning a client applied all of the mixing and effects they wanted - product.

The only things that I did which was perhaps a bit different than normal is that

1. I kept it shorter than most reels - knowing that most casting directors will only listen to a few seconds or jump around the track.
2. In my initial segment, I used my own voice as the soundbed by layering my speech into the background. I don't have any background in production or SFX, so this was my way of cheating some ambience into my reel.

I don't know what sort of success others have gotten, but mine has given me enough work to call myself "full-time" and quit my day job.