Doing visudo you get nano instead of your favorite text editor?
Mine is vim therefore I issue:

DEBIAN/UBUNTU way

# update-alternatives --config editor
There are 4 alternatives which provide `editor'.
  Selection    Alternative
-----------------------------------------------
          1    /usr/bin/vim.tiny
          2    /bin/ed
*+        3    /bin/nano
          4    /usr/bin/vim.basic
Press enter to keep the default[*], or type selection number:

Selecting 4 I’m ready to use my full syntax colors when I edit any file.

OTHER DISTROS
Edit your .bashrc file and add the following:

EDITOR=vim
export EDITOR

Next login you will have your VIM working.
If you want to have it immediately and only for this session just type

# export EDITOR=vim

and press enter.

This tip might comes handy when you do a system check and you want to make sure you don’t check the same file twice.

Let’s pretend that our “file1″ is a conf file that needs review. As you can see the output of the command issued below shows that the file was last edited in June.
Today I want to check the file without editing it and make sure next time I won’t check it again:

$ ls -l
total 0
-rw-r--r-- 1 luca luca 290 2009-06-29 16:33 file1

Touch is an excellent tool in this case:

$ touch file1
$ ls -l
total 0
-rw-r--r-- 1 luca luca 0 2009-08-29 19:43 file1

The Modification Time has changed and so has the access time.

If you want to change just the modification time leaving the access time untouched try with the -m option

$ touch -m file1
$ ls -l
total 0
-rw-r--r-- 1 luca luca 0 2009-08-29 19:46 file1
$ stat file1
[..]
Access: 2009-08-29 19:43:45.000000000 +0100
Modify: 2009-08-29 19:46:15.000000000 +0100
Change: 2009-08-29 19:46:15.000000000 +0100

And -a is just for the Access Time.

Another interesting option is -t. It lets you set the time and the date with whatever you like. This is often used to do fishy things

$ touch -t 200701012301 file1
$ stat file1
[..]
Access: 2007-01-01 23:01:00.000000000 +0000
Modify: 2007-01-01 23:01:00.000000000 +0000
Change: 2009-08-29 19:52:26.000000000 +0100