I basically can't.
Plus well, I like it.
I notice Google will only give a site so many answers. Even a powerhouse like BB.com will only get say 3/4 positions on page one - even if they have 7434 pages related to the subject.
Some real longtails, I notice I will have 3/4 of one domain, then 3/4 of the other....basically taking up almost the whole first page. Some once you chuck my Youtube vids in, places I have submitted a snippet of my RSS feeds for excerpts - I take up the whole first page.
I think PR really doesn't matter as far as placings, at least for me it doesn't. Since I got spanked [dropped from 3 to 0 on one domain lol] my search positions have skyrocketed higher...