Mistakes are part of life. Hopefully we all learn from them, then move on, never repeating the worst ones. Here are six top mistakes most investors make that are easily avoided.

1. Using your investment account like a bank. In other words, you have some extra money that you've earned as a bonus or sold a car or some other one time event and you decide to invest it while you determine what you want to do with the money. The problem: if you buy a stock, it may crater the next day and you could lose a large chunk. For example, a war is declared or the price of oil spikes by $10 or some other catastrophe hits. The stock market will react and not in a good way. When you have extra money, put it in the bank. If you don't need it for at least 2 years or longer, then buy some stock. The risk in stocks is too great in the short run to warrant using it as "parking" spot.
2. Not saving enough. If there's one frightening lesson from the last 2 years, it's that anything can happen to any asset class, except cash. Cash has a very real comfort to it. True, it doesn't grow very quickly, especially in these days of low interest rates. But it doesn't disappear either (even if your bank does, but then another bank will take over your deposit, and you're fine ... that's what FDIC insurance is all about). And if you've been one of the unfortunate to lose your job, having the extra cash to live on for 6 months to a year came in extremely handy. In these uncertain times having a year's worth of living expenses saved will be as good as Ambien for sleeping. After you've saved, then invest. Carefully.
3. Not doing enough research. Many investors hear a hot tip, rush to their computer, and buy it. Then buy more (especially if it goes down). They may have heard what the company did, but since a talking head on TV said it was great, they don't really need to know. If he/she said it was good, that's all that's needed, right? Not exactly. Unless you've done good research on a stock, know its revenues and earnings pattern, and understand its business, you shouldn't own it. How else can you know if the stock is rich or cheap? Basic research, nothing fancy or too advanced, can save lots of dollars. If you hear a hot tip, research first, then decide if it's the right stock for you.
4. Not diversfying enough. If you fall in love with a stock (see below) and continue to buy it, no matter the news (or reality), and only buy one or two of these stocks, you're guaranteed to rise and fall with their good or bad fortunes. Also, if they do terribly, you're wiped out. Diversification won't make you rich overnight, but it could keep you from being very poor very quickly. Buying mutual funds is the fastest and cheapest way to diversify, especially if you're just starting out. After you gain more experience in research, you can start building your own portfolio of stocks. But build it with many stocks across several industries, not concentrations in one.
5. Fall in love with a stock. Emotions have a way of getting in the way when it comes to investing. As Warren Buffett described his approach to investing when he started: "I was thinking with my glands, not my brains." Let your glands handle your love life. Use your brains for investing. When you fall in love with a stock, you tend to lose all perspective (just like real love). You buy more when it goes down. You sell other stocks to buy even more. Then you only own the one stock. If it keeps going down, you feel bad, then worse, even cranky. You lose sleep. You keep hope alive, but the stock doesn't return your enthusiasm, especially if it has lower revenues and profits (if any). Be as objective as you can about your stocks. Judge them on their merits, not their promise. If they don't measure up to your standards in terms of growth or milestones, dump them. Yes, even if there's a loss involved. Then take the proceeds and buy another stock that meets your standards. There are plenty of good stocks that deliver solid growth. The ones that don't, don't deserve a place in your portfolio. Stay out of love in stocks.
6. Can't take a loss. Hard to do. Means you've made a mistake. Better to live with a loss and hope for the best than to realize it and move on. Here's how the thinking usually goes: Surely, it will come back. I'll break even, then I'll sell. Give it up. Some stocks come back. Many don't. You can sit on losing stocks for years, hoping for the best but seeing the worst. Take the loss after a stock hasn't performed in one or two years (much faster if it's lost a major contract or had accounting problems). Move on. As soon as you find your next stock, you'll feel better. I promise.

I've made all these mistakes and many more. These are good ones to know, especially if you're just starting to invest. If you can avoid them, you'll save a lot of money.