That’s something which you need to be very smart about. Simply by using rel=”no follow” for external site you will not be able to increase your page rank.
Page rank is something which is based on sophisticated algorithm, which no one other than Google Engineers could explain with 100% accuracy. Yes, all serious SEOs know what it means and what are some possible factors that can help to improve page rank?
One suggestion regarding this is to combine your link building efforts with applying with rel=”nofollow” attribute to all the lesser important pages on your site.
For example the page rank for your home page is 4 and you have 1000 pages
on your site with some 30-40 links going out from your home page. Do you think all 30-40 pages are equally important for you or do you want them to be indexed by Google at all.
Give it a try, you will not loose anything but definitely will gain. It is like applying strong adhesives to all the leakage area which was letting your link juice drip out of the container.