Sometimes

Consider various ratios such as P/E, P/TBV, or PEG. Look at market trends. Be willing to admit you were wrong.

Always

Wholly understand the business you are investing in. Continue to learn. Remember you are owning a part of a business. Perform fundamental analysis. Stick to your plan. Remove emotion from your decisions.

Never

Invest in a security outside of your sphere of competence. Blindly follow other’s opinions. Think that your investments are lines that wiggle up and down on a chart. Chase the “hot stock.” Make decisions based on emotion.