More Tag Modifier
It's easy enough to modifier a WordPress template to include standard code, such as advertising blocks, above or below the content of an article, but it's not so easy to put a block in the middle of your text. There are some plugins for the likes of Google Ads to allow you to do this, which is great, if that's all you want to do, but what if you want to use another advertiser, or just put in something custom? Well, now there's the More Tag Modifier plugin.
This plugin will take whatever is in a file you designate and put it wherever you insert the more tag in your article. You can see an example of this, using Amazon's advertising code, at A Screen Near You, just look at any of the (longer) articles and you see the block appearing in the middle of the text.
Installation instructions are included in the zip file.