"Through wisdom is a house built. And by understanding it is established. And by knowledge shall every room be filled with precious and pleasant riches!"
Always do your homework.
The more you know, the better off you are!
This requires that you keep learning, and pay attention to events that might affect you.
Understand personal finance matters that could affect you. Understand how each of your investments fits in with the rest of your portfolio and with your overall strategy.
Understand the risks associated with each investment.
Gather unbiased, objective information.
Get a second opinion, a third opinion, etc.
Be cautious when evaluating the advice of anyone with a vested interest.
If you're going to invest in stocks, learn as much as you can about the companies you're considering.
Understand before you invest!
Research, research, research!
Experiment with various strategies before you put your own money on the line.
Examine historical data.
Try a technical analysis portfolio, a dividend portfolio, a price/earnings growth portfolio, and any others you think of.
In the process you'll find out which ones work best for you, and finally ...
Learn from your own mistakes ...
But mostly ...
Try to learn from the mistakes of others!