My share selection golden rule is that I must make sure that the company can make more money in the current year than last year and its projected P/E ratio is less than 10.
To ensure that the rule can really work, you must understand the business of the company. You must also look at its profit in the last few quarters to ensure that it has good profit growth prospect.
You must not buy if its projected P/E ratio is more than 10 because in business there are always some unforeseen difficulties.
I have been using my share selection golden rule successfully as shown in the following charts and the lists of top shareholders of VS Industry and Latitude Tree.