How to reduce the size of swap files (Mac OS 10.6) \
I’m writing this answer assuming you are bad with the technicalities. Please tell me, if you are not :)

The only way to reduce the size of your swap is to use apps that use less resources of the computer to run. When thinking of resource-heavy apps, Excel, Firefox and Safari immediately come to mind, especially if you open a lot of tabs. 

Keeping your (underpowered, if you will) computer running nicely will require some effort on your side. Quit apps that are not in use. Close tabs that are not in use. Restart apps if they stop responding.

Also, if you can update your computer to Mountain Lion, please do so, as Safari on Snow Leopard is really heavy. 

And yes, restarting the Mac clears your swap files.