clear (\c) Clear command.
edit (\e) Edit command with $EDITOR.
ego (\G) Send command to mysql server, display result vertically.
quit (\q) Quit mysql.
source (\.) Execute an SQL script file. Takes a file name as an argument.
system (\!) Execute a system shell command.
tee (\T) Set outfile [to_outfile]. Append everything into given outfile.
\c alone could save you a ton of headache. \e's pretty classy, too; just make sure you follow it with a semicolon to execute the command.
http://dev.mysql.com/doc/refman/5.0/en/mysql-tips.html#verti...