I WAS in my early teens when I bought my first stock. At that time, I wasn't old enough to open my own brokerage account, so I got a little help from my dad but I promised to pay him back. Soon after the purchase, I told my late grandfather that I was gunning to be a bona fide investor.

He took my scrawny arms and put his face near mine so that I could see every crease in that weathered face of his. 'There'll be plenty of opportunities to get in at a great price down the road,' he told me. 'But always remember when you board the bus, you must know when to get off the bus.'

His words of wisdom still ring true today. Picking a stock is hard but knowing when to sell can be even harder. Many punters, for instance, enter a trade without any idea of an exit strategy. They take premature profits or, heaven forbid, let their losses run to the downside. Invariably, knowing when to get out correctly if you're wrong and when to take profits if you're right can be mind-boggling.

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