It depends on the service. Usually ones that "guarantee" your ranking within a specific time frame are doing techniques that are considered black hat, and if Google catches on, it's not the service that will be penalized, but your site. You'd be better off doing your own
SEO slowly than trying to get to the top fast and getting pushed back into the 150+ SERP's.
Good
SEO tips include optimizing your pages with relevant keywords in your title and meta descriptions, creating good internal links on your site, getting quality backlinks to your site (relevant to your site's them and not in low ranked directories, link farms, etc.). Blog are also naturally favorited by Google, so just keep producing good content and you will go far.
I just attended an event on social media, blogging, and
SEO trends which shared some great information for both beginners and advanced bloggers. I wrote about the highlights in an article on the
Best of #SMAZ, which includes a full presentation of the 11 Must Do's for Serious Bloggers.