record your estimates so you can see how close you were. If an overall estimate involved calculating subestimates, keep track of these as well. Often you'll find your estimates are pretty good—in fact, after a while, you'll come to expect this. When an estimate turns out wrong, don't just shrug and walk away. Find out why it differed from your guess. Maybe you chose some parameters that didn't match the reality of the problem. Maybe your model was wrong. Whatever the reason, take some time to uncover what happened. If you do, your next estimate will be better.
