There are different categories of back links that have varying value for SEO purposes.  Let’s review those categories so that you can focus your efforts on those back links that will get you the most benefit.  Note that Google does not publish a schedule indicating which back links they rank more highly.  The following information is inferred from test results, observations and common sense.

Bookmark Links

Given that you can buy a copy of software like Bookmarking Demon or SEOnuke which will generate thousands of bookmarks for you in a few minutes, would you is Google rank these types of links highly?  Likely not because these types of links, while originally indicating some social popularity, now are easily obtained through automated software.  Therefore, there is no harm in bookmarking but to do so manually would be up terrible waste of time and should only be done with software.  One of the least expensive and easiest ways to do this is using Onlywire.

Blog Roll and Footer Links

These types of links have limited value mostly because a high number are paid links rather than freely given by the blog author.  As a result, Google does not see these types of links as terribly important.  Once placed, these links will associate with every post on that blog and therefore don’t indicate much conscious attention by the blog owner to the site being linked.

Comment Links

Comment links are useful and have good value when two conditions are met:

a) the site, and even better, the post to which the comment relates, has a high page rank
b) the link is from a post on blog matching the subject matter of your site

What has no value is to get some software that will place 8000 comment links for you in the next 42 minutes because it would be only by chance that any links meet the above two criteria.  Most of the links will be on junky sites and on topic-unrelated posts.

Contextual links

A contextual link is a link in the middle of a sentence of a blog post or other website content.  These links have the greatest value because they indicate that in the process of writing a post, the author thought highly enough about your content, that he linked to it.  The link is conscious, specific and topic-related.  Clearly, contextual links from high page rank posts in your subject area are gold.  There are two ways to get such contextual links:

a) guest post on somebody’s site, meaning supply an article that you write with links back to your main site, or
b) bring to the attention of the website owner that you have some content on your blog that so compelling, that in one of his posts or in his content, he willingly links to your writing because of its value and pertinence to his writing.

It is likely that getting guest post opportunities is easier because it requires the other website owner to simply say yes rather than alter their own writing for your benefit.  You can find a lit of networks offering guest post opportunities here.