What is the most useful strategy in trading?
Actually, unless you can somehow milk the market (through arbitraging, front-running or whatever), the most useful strategy is following the trend. Statistically, the price has more chances to continue moving in the same direction than turn back and start moving in the opposite one at any given moment after the trend has established itself...
But beware, trend is surely your friend, until it suddenly stabs you in the back and steals your money