There are plenty of other ways to negotiate time, too. There's flextime, there are personal days, there's payment for unused vacation time. Another clever way to make more money on the job is to gamble. Now, I don't mean running a poker game in the cafeteria - I mean betting your boss that you will meet or exceed a target. Discuss these compensation items with your boss at review time, or any time there is a change in your company's operating procedures.
Bosses like to reward excellence.
You want a specific dollar bonus for this type of work. Sometimes these things can even get you a raise outside the normal channels. For instance, I worked with a client who had a boss who was receiving criticism from the board for the high turnover in the company. And all this when the board had declared that the top raise would be 3 percent that year. So, when looking to increase your compensation, consider taking extra value in terms of time and bonuses.