brazenness
Well, yes. :D More importantly I would do the following:
i. Be polite when contacting professors; make it obvious in your letter that you're at least passing familiar with their work.
ii. Have some skills that will be useful to the professor. If he uses GAUSS, know GAUSS; Stata, Stata; Matlab, Matlab; etc.
iii. Taking a professor's course will probably help you do (i) and (ii) and if you do well and talk before/after class I imagine that won't hurt. Just make sure you also impress with your performance, if you pull a mediocre grade it's very unlikely you'd get hired yes?