Don't trust anybody; the broker encouraging you to trade more, the trainer peddling an expensive course or the author with a system that allegedly worked for them but might not be right for you.
Never overestimate your intelligence and skill.
Never overestimate your likely Sharpe Ratio. Treat back-tests with several pinches of salt.
Don't assume you'll be lucky. Assume you won't be and plan accordingly.
Don't try anything too clever or overcomplicated.
Don't trade mindlessly. Don't be the guy caught short when the SNB released the peg.
Don't make too many trades. Don't make your broker rich(er).
Don't be too aggressive with position sizes. Know you risk. Only commit capital you can afford to lose.
Don't relax. At some point something in your portfolio will go wrong, in a way you didn't expect. Limit your exposure by diversifying, and keeping risk within limits.
Don't meddle. Find a strategy that works for you and stick to it consistently. Don't be tempted to 'improve' it.