You can't see the future through a rearview mirror
All the math you need in the stock market you get in the fourth grade.
Although it's easy to forget sometimes, a share is not a lottery ticket... it's part-ownership of a business.
Hold no more stocks than you can remain informed on.
There is always something to worry about. Avoid weekend thinking and ignoring the latest dire predictions of the newscasters. Sell a stock because the company's fundamentals deteriorate, not because the sky is falling.
Investing is fun and exciting, but dangerous if you don't do any work.