How to Free up Unused Memory in Ubuntu/Linux Mint

http://www.upubuntu.com/2013/01/how-to-free-up-unused-memory-in.html

sudo sysctl -w vm.drop_caches=3

Or

sudo sync && echo 3 | sudo tee /proc/sys/vm/drop_caches

NOTE: You can use cron jobs to schedule the commands above to run at specific time intervals.