I started out by getting a recommendation from a professor and the professor teaching the class gave me an override to take the class for credit (by contacting admissions). From my understanding it counts just as any elective would credit wise. I'm sure not all schools are the same and other schools may have more restricted access to graduate courses than others (I know I was considered the bottom rung of the ladder as to availability). That would be more of an issue with your specific school. But once I took one graduate course and received an 'A' the others were a lot easier to get into.
Of course my program, quantitative psychology, is a bit of a special case compared to most programs in psychology. Most institutions do not offer coursework in it at the undergraduate level in psych departments (besides the basic intro stats and research methods). Therefore, unless you go into the math/stats department or take graduate courses, you obtain little experience or knowledge of the field. That being said, most programs that you apply to are an application to the program, not a specific professor or research interest. Usually, you develop your research interest within your first year or two after you get a core curriculum. So as I said in my EDIT, my experience may not be representative of psychology as a whole, but I feel it does not hurt and it is a way to make your application stand out above others. As my I/O psych professor put it, a resume's purpose is to get you an interview. This can easily be extended to an application as well. As a final note, taking graduate courses also allow you to judge if this is what you want with your life by providing you personal experience into what is expected of you and by affording you an avenue to meet graduate students (who can provide insight into what it is like to be one).