Are forum & blog comments a viable way to build links?

We all know what the link building methods in the "old days" consisted of...
Blasting a site with tonnes of forum links and blog comment links and waiting for the "juice" to kick in.
Was easy enough and could be automated with software - which is exactly why these kind of links were then made nofollow!
However, while I don't advocate building links in this way - there IS a viable way of building links using forums and commenting on blogs... Here's how it should be done.