Good site architecture is key. Create a tree-like linking structure, limiting the number of external links on each page. Generally, your home page will have the highest PR and PR will gradually drop as you go deeper into your directory / linking structure.
Consistently keep building links. Direct those links to both your homepage and to internal pages. Also make sure every page of your website is accessible within 2 clicks of your homepage.
Adding "nofollow" to low content pages such as your contact page helps distribute your PR evenly. I think your homepage is still the most important though.
You should could have 5,000 backlinks and still have a PR0 if your link isn't placed on pages with PR.
I remember reading in a SEO book some time ago that internal links from the home page don't lose the home page pr. What does everyone else think? In reply to the original question, target back pages which contain important keywords for your business and get back links to them. Possibly also supply internal links to selected important pages.
getting PR on all your pages is quiet hard. I suggest you build backlinks on a few of your pages, make sure that you have DoFollow links to your other pages from there. this way you will spread your PR over more pages
get a backlink from high pr sites. A continual flow of Pr 5 links over 12 months is better than sudden influx of them, this shows that your content is being valued over time. Generally just keep links coming in on a regular basis.
There is no real bullet proof method. Countinuos link building should help you retain pr or even increase it, but there is no 100% method.
It's not easy to retain. You can't stop doing what you have been doing for the good PR, otherwise it will drop easily.
Get more backlinks than giving outbound links. Get involved in networking so that people can help you to promote your internal pages.
It is hard work that never ends, just as you think you are there you drop in ranking and tyhen you have more work to do.