I'm no expert on SEO, but providing my personal experience - yesterday there has been a massive change in website rankings thanks to an unconfirmed update in algorithm by google, my website too went down along with others - the defination of spam link building strategy is pretty clear -
No comments on blogs with anchor texts
No Anchor text Signatures in forum posts
No Mass social Bookmarking
No submission to directories that low PR or PR 0
No Participation in Link Exchange Scheme
No Gibberish Web 2.0 Articles and keyword stuffing.
( courtesy - Google webmasters guidelines)
The Do's What matters now..........
Quality Content on your website
submission to just a few high PR directories ( most of them are paid )
Promotion of your website in a proper way on Social media
..............................If anybody else here know about more about the DO's please contribute to this post and correct me if i'm wrong at any place.
PS - I thought forum posting with signatures was OK until yesterday but my site came down on rankings yesterday so even that method too is spam now.