What mistakes did you make, and what advice do you have for others to be careful?
What are the best and worst ways of storing a seed in your opinion?
Not a seed, but a private key. This is purely because I don't know much about data security and privacy.
Using a Windows VPS to import private keys is the worst idea. It happened when I didn't know the best way to secure my wallet, I deliberately imported the private key that I used in the altcoin wallet application.
About a week after I turned off my VPS and deleted the instance, it turned out that the contents of my wallet had been completely drained. I can't confirm where the disaster came from, but my biggest suspicion is that I imported the VPS. Leaving private keys hanging around online, was the worst thing I've ever done.