You should focus on building backlinks from different sources such as article marketing, directory submission, and blog postings. If you build enough high quality backlinks your pr will increase over time, you should not try to artificially manipulate your pr.
http://forums.digitalpoint.com/showthread.php?t=224828 Pretty much sums up all the different strategies you should use (kingofsanda mentioned the major ones)
If you read 20-30 threads here in the Link Development forum you will get an idea where you can get backlinks from
There are tons of free tips and techniques for link building out there...... check out my blog for some of them: http://linkbuildingbible.com