The extract tool can't help but leave crap all round the edge so firstly, avoid that like the plague!
Other than that i'm afraid the only reliable one is to create tighter paths around the subject (pen-tool all the way!!). Even then though I seem to always need to free-hand the eraser to get things perfect, but a cheaper fix for quicker jobs is to leave it a little room around your subject, pull the selection in a pixel, feather by 1, invert selection and clear. This tidies things but obviously isn't as good as getting a tight path in the beginning.
Hope this helps