Use sudo

1. when you forget to run a command with sudo, instead of retyping the command you could use

# to run the last command in history with sudo
sudo !!
# or you could use if you prefer
sudo !-1
# run the second to the last command in the history with sudo
sudo !-2 

2. To have an interactive shell as root:

sudo -i
# or
sudo su -

3. when you use vim and forget to type sudo

:w !sudo tee %