Jacken describes in detail how you can use WordPress’ Custom Fields to easily set your own quieries:

But wouldn’t it be great if you could use the Custom Fields that’s already in WordPress? Well you can. And here’s how I did it. When writing your blog posting, just add a tag called “ad” in the Custom fields, and then put the search word you want, for example “ipod”. I’ve put the code so that WordPress uses the the codes when displaying a single post.

Click here to read Jacken’s post.