1 - Not knowing wat you're doing, why you're doing it, or how to do it, AND not understanding the rules & regulations governing what you're doing.
2 - Don’t try to out-smart the market, you’ll loose but if you always take what the market is willing to give you, you’ll be successful. Other words, you don’t invest against the trend since the market is always right. And NEVER trade on emotions, once you let emotions in your trades you will loose.
3 - Never invest 100% of your capital into any one security and never have 100% of your capital invested. Never go into a trade without knowing when and where you are going to get out of it. Never let a loss on a trade get greater than 8%-10%, always take you loss and walk away - don't loose more than you need to and don't be afraid to take the loss. Remember you never can get hurt taking a profit. Never average down, but you can average up.
The basic attribute one need going into an investment is DISCIPLINE, Follow your investment and trading plans don't let emotions get in you way.
from THE STREET