One of the most important aspect of your investing life is rebalancing. As you know a portfolio must be diversified in that the portfolio is composed of a wide variety of asset types such as bonds, stocks, etc. Depending on your investment plan and your risk tolerance the different asset classes will be a certain percentage of the portfolio. And in each asset class their would also be some diversification.
Hence it is important that your portfolio composition be as close as possible to that that your plan states. However the economy is not static. Depending on the state of the economy some stocks might increase in value while others decrease in value. This will cause your exposure to these sectors to change. The percentage of the stocks of some sectors of the economy will increase or decrease in your portfolio.
The change in price might also change your exposure to some asset classes. A bull market might increase the percentage of stocks in your portfolio while reducing the percentage of bonds in your portfolio. If left unchecked the asset allocation of your portfolio will change with time and also the riskiness of your portfolio will change. This might thus reduce your chance of reaching your goals.
Thus you need to rebalance regularly. At least once a year is sufficient. Your aim is to reduce your exposure to certain asset classes that have increased in value while increasing your exposure to those that has decreased in value. You will thus return your asset allocation to that that is stated in your investment plan.
You will thus have to sell securities and buy others. This is the most difficult part for some investors because of the cost involved in selling and buying securities. Also some investors may not like selling performing stocks and buying poor performing stocks. However history tells us that a portfolio left to fluctuate with the market always perform poorly.
So keep rebalancing and your portfolio and you will be on course to riches.