1. Trading too often. This largely depends on the size of your asset base. If you only have $10,000 to invest, making about 50 trades a year at $10 a trade is $500, or 5% of your asset base! That is just about one trade a week, but it takes up 5% of your money. So even if you make 12% returns (beating the historical returns of the market), you will only make 7%, well below the historical returns of the market. If you have a large asset base, you can afford to trade more often. But for most people, try to keep commissions low.

2. Selling scared. Sometimes, it is time to face the music and sell a stock that has been a loser. However, you should not sell just because you are scared. You should sell if you think it makes rational, logical sense to close a position. Many times, people sell stocks because the market had a bad day and they're afraid it will go lower or the stock itself had a bad day. This later turns out to be a bad decision when the stock shoots back up.

3. Not keeping any cash on the side. I have to credit Jim Cramer with this tip. This was one of the biggest newbie mistakes he talked about on his Mad Money show. When you are fully invested and have no cash, you can not take advantage of the market when it has a bad day. You are also more prone to panic selling and making other fear-related decisions. He recommends keeping at least 10% of your portfolio in cash, which I think is a pretty good tip.

4. Buying fad stocks. Sometimes, popular cool stocks do well. Examples from 2007 would be Chipotle and Apple, both of which more than doubled (in full disclosure, I own shares of Chipotle currently). These companies are solid companies with excellent growth, so the gains are justified. Often times though, people buy shares in a stock just because other people are buying shares. The obvious example is the tech bubble, when people were paying exorbitant prices for companies that were not even close to turning a profit. The psychology behind people's willingness to buy these stocks was largely because other people were buying them, so they figured people would continue to buy them. That is not a good reason to invest in a company (in fact, it is a horrible one for long-term investing). When investing for the long-term, always make sure the fundamentals are good.

5. Investing in too many stocks. This is another tip I am borrowing from Jim Cramer. Too many of us buy too many stocks and can not follow-up with the companies. We often barely know what we are investing in and have no game plan in regards to the stock. I know I make this mistake often. If we want to diversify, it is easy enough to just buy an index fund or ETF. If we end up investing in 50-100 individual stocks, we effectively become our own mutual fund, but without the resources to adequately monitor the companies we are invested in.