First, we need to add a third party repository to your /etc/apt/sources.list. (Help for this available at AddingRepositoriesHowto) 
deb http://ppa.launchpad.net/amaranth/ubuntu feisty main
deb-src http://ppa.launchpad.net/amaranth/ubuntu feisty main
After adding the repository, we need to update our repositories and install it. 
sudo apt-get update && sudo apt-get dist-upgrade && sudo apt-get install compiz compizconfig-settings-manager 
If you are running Kubuntu then use this command 
sudo apt-get update && sudo apt-get dist-upgrade && sudo apt-get install compiz compizconfig-settings-manager compiz-kde 
Or you won't get any window decorations. 
Run Compiz
To run Compiz for the current session, hold Alt, then press F2, then enter the command 
compiz --replace

Configure
To configure Compiz and associated plugins, hold Alt, then press F2, then enter the command 
ccsm 
Alternatively, for Ubuntu: System -> Preferences -> CompizConfig Settings Manager 
And for Kubuntu: KMenu > Settings > CompizConfig Settings Manager 
Add Compiz to Gnome Startup (optional)
Go to System->Preferences->Desktop Effects and click 'Enable Desktop Effects' 
Troubleshooting
If you ran nvidia-settings, then your xorg.conf lost some info. To fix your compiz window decorations (titlebars) with an nVidia graphics card, run 
sudo nvidia-xconfig --add-argb-glx-visuals -d 24 
, then restart X by pressing conrol+alt+backspace.