I publish guest post guidelines on my blog — feel free to borrow and adapt at will. (There’s also a permanent link to them in the sidebar on my site’s main blog page).
Also, when I run guest posts on Wednesdays, I always start off the post with a little blurb: This is a guest post by so and so. If you would like to submit a guest article, please check out our guest post guidelines.
Having guidelines helps immensely, although I still get a truckload of unsolicited pitches of this variety:
“Dear Madam. I love your blog and read it frequently. I write blog posts about house cleaning, online education, and politics, and I think my posts would be a perfect fit for your blog. Just tell me what you want me to write about and I’ll do it, as long as you include two links to the company that’s hired me to write guest posts for them so they can boost their SEO.”
(Well, they don’t say that last part but they might as well.) Needless to say, these pitches go straight to my Deleted Items folder.