Here are some tipps that should help you to put togehter a good showreel.
1. Show your best work first
Put always your best work first! If the companies don't like what they see they turn it of. So it is on you to keep them in front of the screen. If you don't follow that rule the chances are high that they not just see the end of your reel, they will also not give you the job.
2. Not to long (max 3-4 minutes)
Don't make your Showreel to long. Two till maximal four minutes is a great length. If they did not like your work after that time they will not be interested in seeing more anyway. And if they already liked it you don't want them to change their mind with less good work.
3. Let the pictures „talk“ not the music
Companies hire you for your visual-work and not your audio-work. So don't invest to much work in music. It is maybe nice for other people to see a showreel with nice music but companies are not interested in it anyway and maybe even mute it.
4. Show a big variaty
Show not to much similar work. No company is interested to see 1000 shots in which you did exactly the same. Rather show a wider variaty of your work so they can really see what you can do. That is in the end what the Showreel is for.
5. Showreel just with Breakdown
If you don't add a Breakdown to your Showreel it is worthless. Without any information what exactly you did, nobody knows if you just made a small roto or if you did the whole shot by yourself.
6. Do not show any un-approved work
Do not put any shots on your showreel that have not been approved by the studio!
7. Sometimes less is more
If you have done something and it looks nice in a still frame do not try on any means to put a camera-move, animations or what ever on it.
8. Keep it up-to-date
Try to update your showreel regularly. Only if you show always the best of your work you can present yourself as good as possible.
9. Show what you can do
Show your shots also in different stages and show things that helpt to understand what you did and how you acutally did it. If you for example did the shading, show all the different passes.