In the mainline generic kernel of Ubuntu, there's a module called zram. This is a pretty good trick to add additional "free" RAM to your machine without any change: it creates in-memory compressed block for swap, meaning it eats a bit of your CPU but gives you literally more RAM.
If you're on a VPS for example, having 512 MB RAM, this would actually give you access to 750 MB RAM and would eat just a little CPU from you - I don't even notice it on the Munin graphs.
apt-get install zram-config
Make sure it's started and running:
If you see something like this
# cat /proc/swaps Filename Type Size Used Priority /dev/zram0 partition 62712 6804 5 /dev/zram1 partition 62712 6768 5 /dev/zram2 partition 62712 6744 5 /dev/zram3 partition 62712 6768 5
then it's already running.
Reboot your machine, and voilá.
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