I'm looking for best practices for using the same data in different places without repeating yourself - this could include configuration or business rules.
Example 1. Data validation rules where you want to validate on the client using javascript, but you want to make sure by validating on the server.
Example 2. Database access where your web server and your cronjobs use the same password, username.
Ease of processing and a human-readable solution would be a plus.
Encode your data in JSON. There's a JSON library for pretty much any language you'd care to think of, or if not, it's pretty easy to code one up. If JSON is not enough, perhaps look at YAML.
XML is pretty globally used. Easy to read, easy to write, and human readable. If you're concerned about the space overhead (which you actually aren't if you want human readable) then just compress it before you send it out, XML compresses quite well.
See answers to this question. I think they are applicable here, especially the one with a DSL.
As much hate as they get, for sharing data validation rules, I'm going to have to say Regular Expressions.
I know, I know, everyone hates them, but they are (generally) language-agnostic.