There is a really great write up on how to install and maintain the latest Oracle VirtualBox package on a Debian-based Linux distribution.
Here are the steps in a nutshell:
Add a line to the
deb http://download.virtualbox.org/virtualbox/debian lucid contrib non-free
Download and add the Oracle public key:
wget -q http://download.virtualbox.org/virtualbox/debian/oracle_vbox.asc -O- | sudo apt-key add -
sudo apt-get update sudo apt-get install virtualbox-4.0
That's basically it... Do refer to the link above for extra details and some caveats.