Here are some other ways to build backlinks for your site:
Directory Submissions - targeted links at high PR directories
Blog and Forum Postings - to high PR and frequently crawled sites
Social Bookmarking - a good resource =
Top 30 Social Bookmarking Sites | eBusiness Data | eBizMBA
Classified Ads - for traffic purposes
Article Marketing - submit to high PR directories, could also serve as link baits
Link Exchange - with relevant and authority sites, just don't overdo it
Link Baits - for organic linking
Etc. etc.
Remember that the more quality backlinks you build - the more they would help your site as well in PR building and SERP rankings