I usually buy expired domains to improve PR

Improving page rank is very difficult and you must you SEO as it is the best way in achieving high rank in search engine results. Following are the ways to increase rank
1. Place web content relevant to products
2. Forum posting
3. Blog Commenting
4. Answer posting
5. Writing Blogs
6. Social Bookmarking
7. Link building
8. Article submission
9. URL name

I think SEO software for off page optimiztion are quite help as long we use properly I am sure it will help to get decent ranking