Уоррен Баффетт обсуждал данный вопрос в письме акционерам Berkshire Hathaway в 2013 году. Ниже приведу полную цитату на английском и коротко на русском языке.
Многие инвесторы не могут оценить будущие прибыли отдельных бизнесов, поэтому им подходит инвестирование с низкими расходами в индексные фонды с широкой диверсификацией. Это ответ на вопрос "во что инвестировать".
Важен также вопрос "когда инвестировать". Основная опасность, что инвестор начнет инвестировать во время экстремального изобилия и будет разочарован, когда возникнут бумажные убытки после снижения рынка. Средство для решения данной проблемы - приобретение акций в течение продолжительного периода времени и никогда не продавать, когда появляются плохие новости и фондовый рынок снижается ниже своих максимумов. 

"... Most investors, of course, have not made the study of business prospects a priority in their lives. If wise, they will conclude that they do not know enough about specific businesses to predict their future earning power.
I have a good news for these non-professionals: The typical investor doesn't need this skill. In aggregate, American business has done wonderfully over time and will conticue to do so (though, most assuredly, in UNPREDICTABLE fits and starts). In 20th century, the DJI index advanced from 66 to 11,497, paying a rising stream of dividends to boot. The 21st Century will witness further gains, almost certain to be substantial. The goal of non-professional should not be to pick winners - neither he nor his "helpers" can do that - but should rather be to own a cross-section of businesses that in aggregate are bound to do well. A low-cost S&P 500 index fund will achieve this goal.
The main danger is that the timid or beginning investor will enter the market at a time of extreme exuberance and then become disillusioned when paper losses occur. The antidote to that kind of mistiming is for an investor to accumulate shares over a long period and never to sell when the news is bad and stocks are well off their highs. Following those rules, the "know-nothing" investor who both diversifies and KEEPS HIS COSTS MINIMAL is virtually certain to get satisfactory results. Indeed, the unsiphisticated investor who is realistic about his shortcomings is likely to obtain better long-term results than the knowledgeable professional who is blind to even a single weakness."