http://blog.kissmetrics.com/guide-to-guest-blogging/
http://www.copyblogger.com/successful-guest-blogging/
I do guest blogging to the best I can and what I usually do is:
Find a few of the best blogs in the niche of the client, I look at all elements - check out the social channel numbers, the amount of social sharing on the posts, number of comments, number of posts going up daily/weekly, length of posts etc.
I will then try and make a headline which fits in with the blog but hasn't been done to death.
I will then usually create a list type article (Top 5) because I feel the information is broken down better in this format and allows for skim reading if the user doesn't want to sit there and read 2000 words (I make sure every heading has an image as well).
The client goes in the byline.
That's just me, im not a pro blogger/writer I just try to produce something I would be very proud to put on my own blog.