Michael Jackson on Optimization:

1. Don't.
2. Don't Yet (for experts only).
   

Donald Knuth in Structured Programming with Goto Statements:

Programmers waste enormous amounts of time thinking about, or worrying about, the speed of noncritical parts of their programs, and these attempts at efficiency actually have a strong negative impact when debugging and maintenance are considered. We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil.

Before you send me hate mail, he finishes that thought up with

Yet we should not pass up our opportunities in that critical 3%.

The real trick is knowing if you are the 97% or the 3%.