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Self-promotion is tempting but its the first thing to avoid on someone's blog for starters. Then, a good guest post should be to-the-point, short and made most relevant to the blog you are posting in. 500 words is the maximum if its your first post on that blog. Finally, another good practice is to refer to other posts on that blog as this way you are a natural fit as opposed to just a peripheral addition.
When you are a guest blogger you should make sure you research the blog, read past articles and pick up the writing style of the blog so when you write your blog post you can tailor your writing style to suit the blog.
Avoid grammatical errors and spelling mistakes.