I've read an article from a famous SEO blog, that a good combination of dofollow and nofollow links will help Google decide that you are building backlinks naturally.
[h=1]Hi, I am bendania from USA. There are software tools to help you search by keywords and find high PR blogs (that don’t have the “nofollow†tag on comments, so you get more “link loveâ€) and I’ll link to a free one towards the bottom of this post, look for "Blog Comment Magic".[/h]
Nofollow links can bring traffic, but it has no link juice, it cant help you to increase PR of your site.
Here's a simple step to search for blogs with "dofollow" or have "commentluv" plugin installed. In searching through Google, try this strat: keyword "commentluv" ex. crash diets "commentluv" Your link in a blog with "commentluv" feature is a dofollow one and can help you increase your PR.