Establish your preferred percentage levels of stocks vs. bonds (or stock mutual funds vs. money market and/or bond mutual funds), for instance: 2/3 in equities (stocks, actively managed stock mutual funds, and/or equity index funds) and 1/3 in bonds (bonds, money market funds, actively managed bond mutual funds, and/or bond index funds).
If that allocation were left untouched, it might appreciate at an average annual rate (before taxes and with all dividends and capital gains reinvested) of about 8%.
But, instead, set targets for the equities portion. Excluding the anomalous 1990s, stocks have gone up, on average, about 10% annually.
So, if you begin on about the first of January with $10,000 in stocks, set a year-end equities target of $11,000.
Then, set a higher and lower threshold above and below that, of, say, 5%. Think of your equity values as on an upward glide path, with a five percent tolerance. If they rise more than 5% above or fall more than 5% below that target path, corrective action is appropriate.
In the above example, you begin with $10,000 in stocks and $5000 in bonds and with a year-end stocks target of $11,000 and a 5% threshold, which means that, for the year, equities must be kept within 95% - 105% of $11,000 (or $10,450 - 11,550).
If, by year-end, the stock market has really surged and your equities are now worth, say, $12,550, or $1000 more than your upper target threshold, $1000 of them will be sold off, while the stock market is high, and the proceeds used to buy more bonds and/or money market account shares.
If, on the other hand, the equities markets have taken a terrific beating and yours have fallen to, say, $9450, you'll use $1000 worth of net assets from your bonds and/or money market accounts to buy more equities, while they are low.
The 5% threshold assures you won't be just a day-trader, switching all the time, and that, when you do take action, it will make a real difference: buying low and selling high.
Once you have made a trade based on this system, you will need to set new targets.
If you have just raised a low stock portfolio up to its bottom threshold ($10,450 in our example), this becomes the new beginning level from which to set a new target for the next year-end, i.e. $10,450 + 10% = $11,495.
On the other hand, if you've just reduced your stock or stock mutual fund portfolio to assure the upper threshold ($11,550 in the above example), this becomes the new beginning level from which to set a new target for the coming year-end, i.e. $11,550 + 10% = $12,705.