1. Decide what you are. Are you a long-term investor, day trader, value investor, growth minded? How long do you want to keep a stock you purchase for? A week, month, year, day, doesn't matter? Decide what exactly you want to do to make money in the market, read about and learn from someone else who is good at doing it the way you want to, then develop a specific plan around how you will do it. For example, I can tell that I am a different kind of investor than Tractor. We both may do equally well in our overall portfolios but if I tried to do what he does I wouldn't be as successful as him and vice versa.
2. Follow your own rules. Once you decide what you are and what criteria you will look for to buy something follow those rules. Almost every time I have broken one of the rules I made for myself I have lost money.
3. Invest in things and companies that you know at least a little. Don't know sh*t about banks? Don't invest in bank stocks. Don't know shit about frozen concentrated orange juice? Don't invest in it. Etc......
4. Learn about the theory of sunk costs. Easier to conceptualize and see how it makes sense than to actually make a smart move dumping a loser. If a position I took has taken a loss, I either buy more or sell everything, there is no other logical choice.
If I had to describe myself as an investor I would say that I am a short to medium term value investor. I like to buy my stocks on sale and generally like to sell within a year of the purchase. When Tractor says that stocks are more predictable in the short term based on technical data, something important is to understand that for him that is true. For me, I just don't get it when it comes to the short term and I would be terrible at guessing it, for me, time is my friend. I say this to reiterate, go with your strengths and what you find you are good at. Not all of us have the same strengths and weaknesses.