Having recently cut my first proper cinematography reel, and having seen a huge bump in the volume of work I've been getting thanks to it, I feel I can offer some advice on this front:
- Where possible, I'd suggest cutting down little sequences of shots that show continuity of lighting and camera movement within a scene (i.e. cut in through your wide/mid/close-up/reverse in quick succession - assuming you can do so smoothly) it shows you can light and block a scene rather than just a single pretty shot.
- Keep it between 2-3 minutes TOPS, you'll have to kill your darlings, and finding music to match is hard, but it needs to be short, sharp and able to hold people's attention.
- If possible, cut the reel to your music, having the cadence of the edits sync in with the cadence of the music just makes a reel feel more polished and complete.
- As much as possible contrast different looking scenes against each other, it looks like you have a broader range.
- And I totally agree with Michael - don't let shots that were amazing technical achievements on set (and that you're therefore particularly proud of) creep into the reel just because you're proud of them. If they don't look great - leave them out.
Hope that helps.