1. Not Knowing the Basics. When I started investing the internet was just really starting to take traction so for most, if you wanted to learn something, you went and found a book. Well, I bought one book and thought I knew everything I needed to know to get started. Come to think of it, I just skimmed through the book and got to the fun part on where and how to buy stocks. Please educate yourself, there are a ton free information available everywhere.
2. Terrible Asset Allocation. I had no idea what allocation was and in my early years I was primarily buying tech stocks... and we all know how that went. Always diversify across many industries so as not to expose yourself to one bad sector and reduce your risk.
3. Keeping Losers and Selling Winners. I was really bad at this. I had no plan of exit for a trade if it went up or if it went down. More often than not I would sell a high flying stock without first researching the fundamentals of it. Another thing I did frequently was get so attached to a stock that I could never sell it even if it tanked to nearly nothing. Always have an exit strategy like setting a price target to get out of a stock whether it goes up or down and stick to it.
4. Cost Averaging Down. This type of investing without expert knowledge is a recipe for disaster. I would often times think that the price could never go down more so I would do the crazy thing of buying more stocks based on "gut" instincts. Always do research as to why the price is going down and make a clear defined decision why you need to buy more of it.
5. Investing Without A Goal. I really did not have a defined goal for my investing. I haphazardly invested in all kinds of things with no rhyme or reason or purpose. Now I set investment goals like having a diversified portfolio big enough to pay for a very comfortable early retirement. My retirement age goal is 55.
6. Being Lazy. A lot of times, due to not wanting to read, I invested in mutual funds that had high expense ratios. Avoid those by doing a little research work. I used to quickly scan morningstar reports and buy the latest and greatest fund that had those shiny 4 star ratings. Those ratings are based on past performance. You need to get your research done and invest properly.