Programming is the art of doing one thing at a time.
This applies to your code and to the act of writing code. Methods should do one thing well. At a single time, you should be doing one thing well. Do tests, refactoring, and new features individually and you'll benefit.
A few others that are more life-quotes, but I think apply:
Be regular and orderly in your life so that you may be violent and original in your work
Small things, if not corrected, become big things, always.
I like the classics:
Shaving 1ns off code executed once or even 100 times probably isn't going to matter. Plus, your intuitions of where to optimize are bad.
A recent one I got from Working Effectively with Legacy Code:
This applies to your code and to the act of writing code. Methods should do one thing well. At a single time, you should be doing one thing well. Do tests, refactoring, and new features individually and you'll benefit.
A few others that are more life-quotes, but I think apply:
And don't forget the revisited version: