Anybody have a clue how to change the URL structure of wordpress.

At the moment it is:

www.domain.com/parentcat/childcat/postname

and I would only like it to display

www.domain.com/parentcat/postname

as the URLs can get REALLY long and I thought I would be better for SEO.

I have posted in the wordpress forums a bunch, but I never get a response If anyone has any suggestions it would be really appreciated.

Thanks