If I advise you to eat only 1500 calories a day, not to eat any sugar and to exercise two hours every day, how much weight will you lose? The answer is: not much.
Quite simply because you're not going to follow such a diet. The best diet is the one you'll really follow. And the best best diet is the one you can follow over the long term, not just for a few weeks when summer approaches. Feeding choices that are reasonable and moderate are more effective than extreme diets, since you will not follow the latter for a long time.
Investing is similar. A strategy which is good in theory, but which you will not follow in reality is not a good strategy (at least not for you). This is why the strategy must take into account your circumstances, but also your personality. The best strategy for you is the strategy as close as possible to the ideal strategy, but which you will apply through time (which may mean decades).
For example if you are thirty and prepare for retirement, the ideal strategy is likely to be something like three quarters of your money in stocks and a quarter in bonds (one may even argue that in theory you should hold only stocks). But if market crashes may scare you into giving up at the worst time, then this strategy is not good for you. The best strategy for you is to have an equity allocation as high as possible without risking to panic, perhaps 60–65%. You must get as close as concretely possible to the ideal strategy. You are better off with a good strategy that you do apply than a great strategy that you do not apply.
One difference between investment and food is that the very concept of dieting corresponds to something you do for just a few weeks. Financial investments, on the other hand, are inherently long-term. Whereas reducing calories and exercising for a few weeks or months is always a good deal, having a large equity allocation for a few months is completely different from a large equity allocation over the long term. So with a diet supposed to last a month, you can always consider that if you last two weeks it's still worth taking (or losing in the case of weight). But with investing it is crucial to have a strategy (including an equity allocation) that you can maintain over the long term.
With investment as with food, doing things well gives results. But only if you really apply your good resolutions. And it must be taken into account from the start: you cannot say "I invest three quarters in shares, even if it means giving up at the first price drop". When determining a strategy, when choosing an asset allocation, you must —from the start— come as close as possible to the ideal strategy, while ensuring that you can stick to the plan over time.
