I found Unity to be very memory consuming and sometimes it hangs the whole system. So here comes xfce - A lightweight Desktop which is fast and uses less system resources. you will find it much like gnome 2.x. It comes as the default UI in Xubuntu. But it can also be installed along side unity in ubuntu. Let's start!.
How to Install xfce in ubuntu
1. You can install xfce from the following command. (this will install just the xfce desktop and applications will remain the same)
sudo apt-get install xfce
2. You can install the whole xubuntu desktop (This will kind of turn ubuntu into xubuntu with default xubuntu apps/softwares)
sudo apt-get install xubuntu-desktop
I prefer and recommend option one i.e install only xfce and use your default applications.
After installing logout and select xfce Session by clicking on the ubuntu logo (as shown in the image below) and login you can switch back to unity like the same.
If you find any problems with the screen brightness shortcut keys then installing
power-manager should fix it.
sudo apt-get install xfce4-power-manager
if you're not be able to use the PrintScreen button.
first install screenshooter with the following command
sudo apt-get install xfce4-screenshooter
then goto keyboard shortcuts and add PrintScreen button to command like shown in the image below
you can map Alt+Ctrl+T to launch terminal too.