In reply to bdgm:
And once you know that, you will be able to become staggeringly wealthy. In fact, most regular folks and many professionals tend to buy high and sell low by letting their emotions dictate what they do.
Nutkey's comment is exactly right - when everyone is panicking and there is the sound of wailing and gnashing of teeth, that's the time to buy. When everyone is recommending things, that's the time to move to cash.
While I hate to quote Joe Kennedy (a noted appeaser prior to WWII) "You know it's time to sell when shoeshine boys give you stock tips"
But when are these times? Precisely? No-one knows and so the best protection is diversification between low risk things and high risk things (stocks and shares - Is there really such a thing as a "stocks and shares ISA"? That's uncommonly vague). There is a vast amount of data showing that if you pile on more risk beyond a certain point, you actually don't get a better return in the long run. Although you might be lucky in the short term.