Read the bash man page (if that's your shell) constantly.
Readline and history and the editing commands are huge, as frieza mentions. (Though the default histsize is 500 putting -
- in your configs is useful.
Spend time on your ~/.bash_profile or however you configure your shell.
The man page will tell you about shopts and your configuration can set them up. In my ~/.bash_profile, I have 'shopt -s checkwinsize cmdhist extglob histappend histverify' and maybe should add 'cdspell' and others.
One of the coolest things there is is the semi-colon. The command line is not just for 'ls'. As a trivial example, I was in ~/.icons in aterm looking to add a calculator icon to Ice's little systray thing. So I typed
for f in `find -name *[Cc]alc*`; do xzgv $f; done
and got a slideshow of all my calculator icons, regardless of what subdir they were in. The command line kicks whether in console or a GUI.
You can get a bang out of bangs. '!cat' will re-execute your last cat command (and !wh will execute your last wh-atever command) or, with 'histverify' on, will load the command and you can choose to enter or edit it.
The Linux console/bash is just a blast.