What can you do from here in California even before you go to the country?
Decide on whether you want to specialize in the Spanish as used in Spain or Latin American Spanish. I made a personal decision to focus on Mexican Spanish.

Here are some of the materials I have found helpful in trying to keep up with my Spanish:

A. Wikipedia and Google in Spanish

B. Bibles and Devotional material

1. Read your Bible in Spanish.
Bible Gateway: Nueva Version International

2. Spanish Language Gospel Resources
Berean Christian Book Stores
Case Bautista de Publicaciones Editorial Mundo Hispano
Editorial Unilit

3. SPANISH--Thru the Bible [available through iTunes]

C. Mail-order catalogues; Department store websites

Liverpool Department Store
Sears Department Store
D. News: sample websites

Kidon Media Link
SPANISH--News and Newspapers (MIT Libraries)
Google News (Mexico)
Proceso magazine
El Sol

El Universal
E. Television: sample websites

CNN en EspaƱol
F. LibriVox Audio books in Spanish

G. Books I have found useful:

Barron's Spanish Vocabulary
Teach Yourself Spanish
Cassell's Colloquial Spanish
Berlitz Latin American Spanish Phrase Book and Dictionary
Berlitz Spanish Vocabulary Handbook
Mastering Spanish Vocabulary: A Thematic Approach
Oxford Spanish Wordpack
501 Spanish Verbs
2001 Spanish and English Idioms
Spanish for Dummies
Harper Collins Spanish Concise Dictionary