Find a language partner that will speak with you in Spanish for a few hours a week. Preferably, your partner should be a native Spanish speaker in order to help you learn idioms and common phrases and to help you perfect your accent.
Listen to as much Spanish music as you can. Write down the lyrics and sing along to learn new words.
Watch movies and television in Spanish. Exposure to the language, even if it is just background noise, helps to cement the vocabulary you already know in you mind. Put on subtitles in Spanish to help you follow the dialogue and in English to help you translate.
Study for a semester or for a year abroad in a Spanish-speaking country. Try to find a college that teaches in Spanish. If you are forced to use only Spanish with a host family or Spanish students, you will learn faster.
Take advanced classes in Spanish history, culture and literature. Advanced classes can help you learn the language through other interesting topics. Once you've completed all of the grammar coursework possible, continue with other subjects in Spanish.