Google finally launched dayparting which can be a very effective measure for cost savings and improved ROI.
Be careful when implementing as you may decide that since sales are slack during a certain time of the day that your ads should be turned off during those hours, when those clicks could actually lead to sales during a repeat visit during a different time of day. Ensure that your analytics package is tracking the time of the click and not just the time of the sale to ensure you're dayparting most efficiently. I use Omniture Sitecatalyst as well as Google conversion tracking. Out of the box, Omniture doesn't tag the visit with the time of day, so I use a custom metric to assign the hour of day, 21:00 for example, and measure the revenue/conversion rate per keyword off of that metric.