3) NEVER INVEST IN A BUSINESS THAT YOU DO NOT UNDERSTAND
I find this advice the most helpful, I invested in a few projects that I heard and there was buzzwords flying all around, but I didn't knew exactly why they will be useful. And of course they were just pump and dump coins. I guess I will try to learn from mistakes.
It would be interesting to get a poll to find out the % of investors who have done this. You're honest enough to admit it, but the poll will not be accurate unless people can anonymously answer.