There are many factors that tend to promote good sustainable long-term investment results. One of the more important involves having realistic expectations about what a well-thought out investment strategy can do for you as well as its limitations. This will help you stick with winning strategies during inevitable periods of lackluster progress from time to time.
No investment strategy, no matter how good, works all the time. On the other hand, no investment strategy, no matter how bad, will fail all the time. Even successful investment strategies will generate alternating periods of strong and more lackluster progress (including possible losses). A good investment strategy over time should produce 6 or 7 winners out of every 10 investment selections, and minimize the damage from the 3 or 4 losers, thereby generating satisfactory long-term returns for the entire portfolio. For example, the legendary Peter Lynch admitted his batting average was around 6 out of 10; his spectacular investment record was the result of the gain from his winners greatly outweighing the loss from his failing investments.
Having unrealistic expectations regarding what a well-thought out investment strategy might achieve for you will likely hurt your long-term results. For example, if you thought a good investment strategy always achieves a 100% batting average (yes, lots of people think this way), then you are likely to abandon a good strategy going through the occasional and inevitable lagging period.
For instance, it has been widely reported that many shareholders of the Fidelity Magellan Fund under Peter Lynch's tenure actually failed to make any money because they abandoned the fund during periods of lagging performance (i.e. sell low) and chased the fund after periods of out-performance (i.e. buy high). Yes, even Peter Lynch's spectacular career contained some forgettable periods of lackluster progress.
When judging the efficacy of an investment strategy, you need to be aware that even the best strategy will involve a certain amount of short-term emotional pain from time to time. Knowing this in advance and being psychologically prepared for it will help you stick with winning investment strategies even if they don't and can't "win" all the time.