In anytime I see someone code inefficiently. Here are three of the most common mistakes, followed by a better way to do the same thing.
Bad: cat somefile | grep something
Better: grep something somefile
Why: You're running one program (grep) instead of two (cat and grep).
Bad: ps -ef | grep something | grep -v grep
Better: ps -ef | grep [s]omething
Why: You're running two commands (grep) instead of three (ps and two greps).
Bad: cat /dev/null > somefile
Better: > somefile
Why: You're running a command (cat) with I/O redirection, instead of just redirection.
Although the bad way will have the same result, the good way is far faster. This may seem trivial, but the benefits will really show when dealing with large files or loops.