The saying, "Don't put all of your eggs in one basket," couldn't apply more to saving for retirement. Financial advisors, investment bankers and economists will all tell you that the more diverse a portfolio, the safer it is. A person heavily involved in just one type of investment is more vulnerable to financial problems if the markets associated with that investment tank.
The most common diversification suggestion is to divide a portfolio among stocks (which can offer big pay-offs but can also be high risk) and bonds (Treasury bills that offer little to no risk, but pay out less than stocks). Depending on who you talk to, you'll hear different percentages for splitting your portfolio between stocks and bonds. One good rule of thumb is to keep your bond percentage close to your age adjusting as life goes on; so if you're 30, about 30 percent of your portfolio should be in bonds. By the time you retire, 60 to 70 percent of your portfolio should be in bonds.
Don't stop with just stocks and bonds when diversifying your portfolio. Look for other ways to spread risk among your investments. Investing in largely unrelated sectors, like pharmaceuticals and telecommunications, is a good idea. You should also consider investing in economies throughout the world, rather than companies in just a handful of countries or a single region.