Don't Invest Without A Portfolio Plan.
If you invest based on how the wind blows, you'll get clobbered when a storm hits.

Don't Buy Any Securities Without Doing Research. 
Know what the company does, and what it's future prospects are.

Don't Buy Based On The Advice Of A "Wall Street Wiz" Without Checking Their Background. 
The brokers who give you a call may be registered with the regulators, but they make money based on how many transactions you make, not how well your portfolio performs.

Don't Invest The Rent Money. 
If you have money for rainy day or emergency expenses, don't put that in the market, you may have an emergency when your investments are at an all time low.

Don't Let The Market Dictate When You Invest. 
You should invest regularly, not just when the market is "hot". If you invest only when the market is running hot, you'll pay a premium for the stock and when it goes down, you'll take a bath. On the flip side if you don't invest when there is a market downturn, you'll miss a chance to get some undervalued stocks at a good price.

Don't Buy On A Tip Or A Rumor. 
The quality of the information is usually questionable, and the hallmark of these tips is that you have to get in right now before the rest of the world finds out. Remember: "It's not the timing, it's the information." By all means check out the company that you've heard great things about. Be as thorough as if you had all the time in the world. Invest in companies, not tips.

Don't Hang On To Dead Weight Because Daddy Did. 
People have sentimental reasons for hanging on to stocks long after they have proven to be as valuable as pocket lint. Cut your losses. 

Don't Buy Penny Stocks Thinking You Can Double Your Money Easily.
Penny stocks (or very cheap stocks) are cheap because the rest of the market doesn't think they are worth much. Research carefully to determine if you're really getting a bargain, or if the stocks are cheap because they really aren't worth very much, and aren't likely to increase in value. 

Don't Forget About Uncle Sam. 
The Capital Gains Tax punishes people who trade rapidly. If you held on to the stock for a year, the top federal tax rate is only 20%. But, if you hold securities for less than a year, they will be treated as ordinary income and taxed at twice the capital gains amount. Here's some good news: if you are taking a bath on a particular stock, you can dump it before December 31st to offset any gains you've gotten. This is because your tax bill is based on your total gains. End-of-the-year dumping is a common practice. So, get out as early as you can as prices typically drop in the last quarter of the year.