If you're looking from an SEO perspective, for largest possible reach and highest possibility of attracting links, you may want to look at:
Where your post is going to be placed
What the Domain Authority is of the website it will be placed on
How popular the site is on social networking sites/how much social interaction they produce - This will tell you if the site is actually one that people are still reading. A lot of the sites on guest blog posting sites are made specifically for accepting guest posts just to give people links. I recommend staying away from those. They're glorified blog networks. You will not get much benefit from those--no traffic, no link juice, no good
To find niche specific sites that are looking for guest authors, use some of these queries in Google:
- Keyword + intitle:“write for us”
- Keyword + intitle:“contribute to”
- Keyword + intitle:“submit” + inurl:blog
- inpostauthor:guest + keyword
- inpostauthor:”guest blog” + keyword
- inpostauthor:”guest post” + keyword
Here are some good resources from a post I did a while back
Here are two good guides that can help you begin:
1.
The Ultimate Guide to Advanced Guest Blogging - YouMoz | SEOmoz
2.
http://www.righthatseo.com/guest-blog-for-the-best-blog
And since Ann didn't do it herself, I'll put the plug in here for her

A good place to start building relationships is
http://myblogguest.com
Do these help answer your questions?
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