What is the most effective use of audio only for language learning? -- for example, while commuting.     -Brian

There are audio-only programs for learning language (as you probably know), such as Pimsleur.

Personally, I do not like material that is explicitly for language learning, when it comes to language learning. I think the more you focus on the learning/studying aspect, the less you focus on the true goal, which is authentic communication in the language.

Here's what I'd do in the car: get an audio book in Spanish -- a legit audio book of a book that you actually want to listen to (or maybe already have listened to or read in English), and listen to it in Spanish.