In particular, as some other comment said, sit someone (anyone) and look how s/he use the software with minimal (without) instructions. Put your hands behind your back and try to be as quiet as possible. Fix the problems and repeat, preferably with another person.
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I recommend all the post of http://www.joelonsoftware.com/ in the section “Software designer” (down, in the middle of the page).
The first article is “Controlling Your Environment Makes You Happy”: http://www.joelonsoftware.com/uibook/chapters/fog0000000057....
In particular, as some other comment said, sit someone (anyone) and look how s/he use the software with minimal (without) instructions. Put your hands behind your back and try to be as quiet as possible. Fix the problems and repeat, preferably with another person.