For example, if you plan to clean your bedroom for an hour, set the alarm on your cell phone. Stop working on the chore at hand when the timer goes off, but not until then (unless you complete the task).
Playing "beat the clock" can be highly motivational. Plus you're likely to get more done.
For particularly troublesome tasks, set your alarm in smaller increments - even 15 minutes - and take a short breather when your alarm sounds before you re-focus on your task. In particular, you may find that physically challenging tasks are much more manageable if broken into smaller, more do-able "pieces."