From Birnam Designs Wiki

You can set how partial Linux is to swap vs. physical memory in /etc/sysctrl.conf. There is a variable that either exists, or that you can append to the file, called vm.swappiness=x, where x is a number between 0 and 100 that determines how much your system will lean on swap. A value of 0 means that your system will only use swap when all physical memory is used up.