I have 4 short advices for everybody. Maybe they are more about trading than investing, but investing involves a little trading.

1. Be patient in waiting for the right moment that is relativly riskless to buy or sell.
2. Stay calm. Even if you had made a mistake its not going to help you to get mad about it.
3. Believe in your first thought. Mostly its the best idea you can get, if you stayed patient and calm that is.
4. Stick to the plan. Its not helpful to change your plans as its usually emotionaly driven.

You just can not go wrong with those ideas. Those are plain and simple and maybe obvious to many of you but I think thats the best advice anyone could give when it comes to trading/investing.