Guests post offer an opportunity for bloggers to get the word out about them. You get to let readers see your content, and it drives traffic back to your site too. I have guests posts on my blog, but had to take down my submission page because of the solicited requests from companies trying to build backlinks for their clients.
Tip 1: If you approach a blog to do a guest post, make sure you have READ their blog. Don't submit a request about "food" and their blog is about marketing. I get this crap all the time and now only do it by invite only.
Tip 2: Make sure you understand that blog's audience, their style and the topics discussed. You want to tailor your post that meets their expectations and their readers.
Tip 3: Write a better guest post than you would for your own blog.
Tip 4: Get to know that blogger before asking to write a post for their blog. I will not accept submissions from anyone I don't know. I don't trust you yet and my readers don't know you from sh-- so why would I?
Use common sense when asking to guest post on another blog. GET TO KNOW THEM! I can't stress that enough.
Commenting on other blogs is priceless because you are networking with other bloggers and they are getting to know you too. People don't realize the benefits of commenting on other blogs as well as guest posts.
Comment first, guest post last. Build up your reputation online before jumping in to do guest posts. If people don't know who you are, what makes you think they will let you guest post on their blog?