Many people believe that for any selection of stocks it is impossible to "beat" the market, that is, to expect to invest more successfully than mechanically following an index covering the whole market; i.e. the Efficient-market hypothesis.
This automatically implies that it is, for all means and purposes, impossible to predict the market in any way. It does not matter how obvious a forecast is, or how prolific the oracle is. It has been shown time and again that forecasts are not reliable. And if they are, then it is already too late - that is, if you and me as regular investors are absolutely sure that the market will crash, then the crash has already occured. The current stock exchanges are incredibly fast, there is no way at all that a normal customer can react fast enough to make any change.
(Note that all of this excludes insider knowledge, obviously - for example, say you know of a certain decision about a company being made public in a week, this gives you a worthwhile heads-up. Or if the heads of a state learn of an upcoming pandemic from reliable sources, and trade their stocks based on this information before informing the public, then they are, indeed, able to "beat" the market. Therefore, insider trading is generally illegal, and is not what this answer is about).
There have been many occasions in the past where, for example, the market seemed "overheated" and people were predicting a crash coming soon. This same situation would then go on for years, while the market did not care and increased continuously. If you had invested, you would have made a nice profit. By not investing you lost it. The opposite is of course true as well.
A valid investment strategy thus is to invest always, no matter what the market does, and in a cheap index instrument which follows the whole market. No forecasts whatsoever are needed. This is a long-term strategy, obviously; crashes will happen, and you will simply sit them out. If you want to know more about the theory behind this thinking, check out The Intelligent Asset Allocator, a nice allround look into these topics.
A nice psychological side effect is that if and when a crash happens, you can be pretty happy about it - maybe you have some cash in the bank and can quickly push it into your index of choice; the assumption is that long-term the index will rise. You can decide for yourself if this is a fallacy or not (after all, you never know when the bottom of the crash has happened), but it is healthy for your psychological state. This may sound funny, but it is in fact incredibly important. After all, a scared investor dumping all his stocks during a crash is 100% sure to lose. Being able to sit out these kinds of down-and-up movements is a very strong instrument in itself.