Clear memory cache in Linux

Read this article

http://linux-mm.org/Drop_Caches

Basically, run commands
    sync
    sudo echo 3 > /proc/sys/vm/drop_caches

If you want to control "swappiness" (to what extent apps are swapped), read following posts:
http://kerneltrap.org/node/3000
http://lwn.net/Articles/92892/

Basic command: sudo echo 0-100 > /proc/sys/vm/swappiness