Anchor Text Links In Forums

I’m thinking about posting on forums with a link going back to my site and I would like a professional opinion to the following:

1. I will not just put my link for the sake of getting a back link to my website, I will participate in the discussion to the best of my knowledge, I will only post in relevant forums where my knowledge on the subject is good.

2. What is classed as a relevant link? Should my keyword be in the URL? Title? Content? Should my keyword be in Bold? Should there be a H1, H2, H3 tag? What makes a relevant link?

3. Anchor Text. Say for instance I’m selling DVD Box Sets, if I was to use “DVD Box Sets for Sale in London” as an anchor text, that would include several of my keywords in 1 sentence, but will Google only rank me better for that whole sentence? Or will all of my keywords included in that sentence rank better?

4. Should I only be concentrating on DO follow forums, forums that only have a high authority?

I know the above just sounds like common sense but I just want to have a strong understanding of forum posting.