I seem to get the same feeling of accomplishment when I cross off some trivial errand as when I accomplish something major. The end result is that trivial errands get done, while the important tasks often get left behind.

You could always exploit that by adding more things to your to-do list, such as adding tasks to break down other tasks into even more tasks. ;-)

There are a few time management systems I know of that actually do have built-in adjustments for this tendency. Mark Forster's Autofocus system allows you to cross of a task after spending as little as one minute on it -- you just write down the next piece(s) at the end of the list, even if all you did on the task was to break it down into smaller pieces. And The Pomodoro Technique has you break large tasks down, or combine small tasks up, to form units called "pomodoros".