As just about everyone who's got a blog or website learns pretty early on, three of the best ways to get those all important backlinks to your site are via article bio-boxes, forum promotion (with dofollow signatures) and commenting on dofollow blogs.
From what I can tell, articles will give the best SEO value per backlink. Not sure which comes next out of the other two. I think it's probably the blog comments. In any case, I do prefer the forum marketing.
This is because blog commenting can be more time consuming than participating in forums. Firstly you have to find the blog, then find the post that you sincerely want to contribute to. Finally you write your comment.
Also, blogs are usually written by just one person, so many of them are only updated occasionally. You can return again and again to a quality blog and not find a post you can comment thoughtfully on for weeks!
On the other hand the content in forums is contributed by hundreds, sometimes thousands of people. So if you find a nice big active forum, you can keep coming back and there'll almost always be lots of new stuff there to comment on and contribute to. You just go to the relevant section and scroll down through the list of threads to see what catches your eye. Because of this, you can build backlinks far more quickly.
There's another advantage that forum promotion (and article marketing for that matter) has over blog commenting: You can go back to what you've written and edit it, including the links back to your site. You can't do this with blog comments. Once you've posted your comment on someone else's blog, that's it!
This may not seem like a big advantage. But it definitely comes in useful if you want to launch a new website or blog. If you've got dozens of posts up there are you've got a very useful instant SEO tool. You just log into the forum, go to your account, then add the new link to your signature. Same with your article bio-boxes. While some of the big article directories allow a maximum of two bio-box links, many of them allow three. If you haven't used all these then you can just put the new one in for each of your previously submitted articles. If you have used them all then you can just replace one of the other backlinks with the new one.
If you have a really large number of these up already then it's probably not a great idea to do them all at once since Google might not approve! But if you spread them out a bit, particularly if you've done some other incremental link building in the past, then it shouldn't be a problem. And it will definitely help with SEO.