All estimates are based on models of the problem. But before we get too deeply into the techniques of building models, we have to mention a basic estimating trick that always gives good answers: ask someone who's already done it. Before you get too committed to model building, cast around for someone who's been in a similar situation in the past. See how their problem got solved. It's unlikely you'll ever find an exact match, but you'd be surprised how many times you can successfully draw on other's experiences.
