I have wanted an automatic paint shaker for a while now but they had bad reviews and were expensive. I found this post on CMON that led me to this video (embedded below).
I realized that I had a sander much like the one he was using that I had paid $20 for. I gave it a try and it works great. Mixes paints even better than shaking.
1. Get a handheld sander.
2. Apply a Crown Royal bag or piece of felt to the sanding end.
3. Place paint pot against the sanding end.
4. Run sander.
I love this method and this paint mixer/sander is going to be a staple of my hobby forever. My only regret is that I did not discover this sooner.