link back from other site is no 1 point, the other is website speed load and quality content, and more additional is age of the domain, but the last point is optional
as for me.. blog commenting is the fastest way to get backlink.. building backlink need hard work.. you just need to be more hardworking..
Getting backlinks from high PR websites. I would say getting an PR increase by 1 less than 2 months is considered fast.
Exchange links with higher PR rank sites. This will definitely cope up your pr sooon. Try to submit in high PR rank directories and concentration on quality backlinks.