PR is just a measure of the amount (and quality) of your inbound links. What you do not want to do is rush out and start buying high PR links, Google does not like it when your website goes from "zero to hero".
If your site is new then it is highly likely that your are suffering from the "sandbox" effect - this is where Google automatically devaules a new website until it thinks it can "trust" it - how long you stay there is really a "how long is a peice of string" question...
My advice would be to start a slow / long term link building strategy - submit to lower PR web directories (free ones) and you will soon see your PR jump to PR3 / PR4 - this is what I would call normal.
To get higher is where you need to start getting better quality backlinks - these you will need to pay for usually but make sure you pay for links from relevant websites and not just any old website.
You need to walk before you can run
