Learn how to set aside time each day to devote to learning Spanish. This is the most important thing to remember when learning any foreign language. Devoting two hours each Sunday to Spanish is much less useful than devoting 15 minutes each day.
Learn as much as you can in the classroom. The reason why many people do not leave high school knowing Spanish is because they focused on the grade and not on the learning. Speak only Spanish in the classroom. Talk to your professor in Spanish for five minutes after class, visit her during her office hours to talk in Spanish, and write her an email every once in a while because she is your best model for good learning.
Learn to do your life as much in Spanish as possible. Put your Facebook and your email accounts in Spanish. Change your cell phone and your Wii to Spanish. Watch movies in Spanish (with English subtitles if necessary), or even better, watch Spanish movies (many Internet sites have Spanish movies to stream to your TV).
Learn how to enjoy music in Spanish. Alejandro Sanz and Ricardo Arjona are great singers to start with. They sing slowly and are easy to understand. On some online sites you can listen and read the subtitles that users have submitted.