There are a few timeless laws you simply have to follow if you want to make money investing.
You can do everything else wrong but if you follow these laws you are virtually guaranteed to make money in the stock market over the long term.
I was going to do a lot of research to come up with my best ideas of the laws but then I read a March 2011 paper by a market strategist, and now also fund manager, I greatly respect and have followed for a long time.
He also works for an investment firm that does excellent work. The quarterly letter of the CEO which I recommended in the article The only quarterly market letter and worth reading is a must read.