As much as possible try to look for a blog list that has a high PR and a do follow link. There are lots of website that offers lists of do follow blog but you just have pay for it. You just have to deal with because their lists consist of a high pr blog list and a do follow link.
Link exchange with two and three way with high quality sites gives u good results u can exchange link with sites which has good quality back links in a search engine
Link diversity is important if you ask me. And that should also goes with a thorough analysis of the site you want to promote. Site age, site's topic/niche, the competitors in that niche generally, the competitors that targeting the same keyword phrase, etc. So, it is pretty complex I think. Just building backlinks won't do much help especially in the long run, you might get to page 2 or page 1, but will it stuck there? will it even improve? That is the big question if we are targeting very competitive phrase or niche.
All the ways are best.. But for me article submitting, blog commenting, forum signatures and social bookmarking are the best options.
yes you're right but for commenting in auto approve commenting is bad for seo and rank will down cause google mark this as spam activity
yes link building is a good way but u must also do other techniques such as like directory submission,forum posting
Do-follow lists are best ways to use for link building and always use fresh content in blog comments, ad posting, social book marking and article submissions.