BY TED CUSHMAN Contractors today are living through some of the best business times in history. Younger builders may not remember the last recession (1990-91), much less the one before that (1982-84). But experienced pros say the business cycle has always been part of their working lives. "As far back as the 1950s, every recession I remember has been presaged by a boom," says Seattle, Wash., remodeler Howard Portnow, whose father and grandfather were builders. "The worst of times are often preceded by the best of times." "If the economy doesn't turn down," agrees Oakland, Calif., remodeler Michael Luttrell, "it'll be the first time in history it hasn't. The higher it goes, the harder it falls." Forecasting the future. Predicting just when