Add extra SWAP file

First you need to create an empty swap file, the next command will create a 1Gb file:

dd if=/dev/zero of=/mnt/extra.swap count=1024 bs=1048576 #(where 1048576 bytes = 1Mb)
sudo chmod 600 /mnt/extra.swap
sudo mkswap /mnt/extra.swap

Now edit your /etc/fstab and add a the line:

/mnt/extra.swap none swap sw 0 0

finaly activate the new swap file with:

swapon -a