How to Free up RAM On Ubuntu variants

First run the following command

free -m

and find out the available memory
kg@kg-netbook-lubuntu:~/opt$ free -m
total used free shared buffers cached
Speicher: 2006 1739 266 0 50 1300
-/+ Puffer/Zwischenspeicher: 389 1616
Auslagerungsdatei: 2035 6 2029

after that run following commands to free up memory
kg@kg-netbook-lubuntu:~/opt$ sync
kg@kg-netbook-lubuntu:~/opt$ sudo -s echo 3 > /proc/sys/vm/drop_caches

this frees up memory. after that you can run free -m command again to see free memory again.
kg@kg-netbook-lubuntu:~/opt$ free -m
total used free shared buffers cached
Speicher: 2006 631 1374 0 11 193
-/+ Puffer/Zwischenspeicher: 427 1579
Auslagerungsdatei: 2035 6 2029