VirtualBox shared folder for Ubuntu guest

After upgrading my Ubuntu guest OS, the configured Guest Addition and shared folders are gone. So I have to go through the exercise again. Keep this note for future reference:

1, Make sure dkms is up to date:

sudo apt-get install dkms

Reading package lists... Done
Building dependency tree
Reading state information... Done
dkms is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

2, install Guest Addition on Ubuntu:

Click VirtualBox menu Devices | Install Guest Additions..., nothing happened. Then run it manually instead:

sudo sh /media/VBOXADDITIONS_4.0.6_71344/VBoxLinuxAdditions.run

3, Add shared folders by clicking VirtualBox menu Devices | Shared Folders

4, Mount the shared folder to the guest Ubuntu and make it accessible to yourself.

The default mounting point is /media/sf_<the-name-entered-in-step-3>, and is accessible to group vboxsf only. To open it up for yourself, go to Ubuntu menu System | Administration | Users and Groups, then go to Manage Groups | vboxsf | Properties | Group Members, and add yourself to this group.

While there, also change the account type to Administrator. It's not a required step, but I like to have maximum permission.

Alternatively, you can also do it in command line, which should work in all Linux distributions:
/usr/sbin/usermod -a -G vboxsf my-user-name