Yes, Pretender, it IS very difficult to move money out of a fund that is giving you 20% gains. However, you have to remember that you are SELLING that portion of the fund that you are moving out, so you are selling high. Conversely, the funds that you are moving into are probably down, so you are buying LOW. It's like getting a discount on your purchase. The problem with investing with the people that I counsel is that they let the market play on their emotions. They feel that they can get a little bit more out of the fund.
In the mid-late 90's, almost anyone could pick a good fund or stock -- things were flying. Then, when it started to take a dive, poeple started to realize that maybe their funds weren't as great as they thought they were. Emotion takes over -- people don't want to settle selling funds at a loss, so they say, "when the value of this fund returns to the point where I bought it, then I'll sell" and that they continue to say that as the fund continues to lose value. Had they rebalanced every year or six months, they would have been locking in those 20% gains and buying the lower-priced allocations.
For instance, suppose that you have an 80% allocation in equities and a 20% allocation in bonds. After one year, your equities have made a 25% percent gain and your bonds have maybe lost 3%. Because of your gains, you now have more allocation in your equities and less in your bonds. So, to regain your balance (rebalancing), you sell the portion of your equities to return to your 80% allocation (even though that fund is giving you 25% returns!) and purchase more shares in the bond fund which lost 3% (but, you're buying those shares at a discount). So, at this point you have SOLD HIGH and BOUGHT LOW.
Now, when the stock market turns, and that 25% turns into a 20% loss, you have moved out a bunch of money into the bond fund, so you don't take as huge a loss. Remember, too, that when stocks are down, bonds are usually up, so now your bond fund is going up 8%. At the end of this year, you may be over your 20% allocation for bonds, so you sell your over-allocation of bonds (again, you're selling high), and you buy more of the stock fund (which is now at a 20% discount because it lost value, which means you're buying low).
Capice?