And of course, you should not join all the forums that come along the way. I have seen people skipping reputed forums because they do not give instant back link from the signature section. This is a sickening mentality. Participating in forum is about helping others and not just getting a link back from them. So, if you wish you can add a link back to your website with Branded Keyword, avoid exact matched anchor texts like plague and you will be just fine.

I don't think a couple of links from the same domain is going to hurt you. If it becomes too many then it loses it natural value.

Sorry I have to disagree. If I am a contributing author of SEOmoz.org and I am getting 120 links for contributing 120 great posts in its blog, Google is not going to penalize me for sure. So, as long as the articles are great, and you are doing guest posts for traffic, you are safe. Rather I would go against the idea of guest posting just for the sake of link building.

Keyword density is passe. Just make  sure you have used the targeted keywords in the title and a couples of times in the body content. But no need to use them as it is. For say if you are writing on Web design NY, you do not need to use the phrase as it.