The commit charge increases when any program is opened and used, and goes down when a program is closed. It will also change when already-running programs allocate or free private virtual memory; for example, with the VirtualAlloc and VirtualFree APIs.
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The commit limit may be increased by either creating additional pagefiles or, if pagefile expansion is enabled, by expanding an existing one. The operating system will expand the pagefile automatically, if possible, when the total commit charge approaches the limit. In such an event a popup window will be displayed stating that "The system is running low on virtual memory."
http://en.wikipedia.org/wiki/Commit_charge
The commit limit is determined by the size of the paging file. If the paging file is enlarged, the commit limit increases, and the ratio is reduced.
http://technet.microsoft.com/en-us/library/cc778082.aspx
So, it appears that the way to reduce the commit charge is to reduce the size of the paging file. This seems to be undesirable. You would be reducing the amount of virtual memory available. This would slow your system because fewer applications would fit in the memory and you would be getting those low memory warnings mentioned above.