Practice, in real life. The best way to learn anything is by practicing little and often. It's the same way you learned to play a sport or musical instrument. Mastery came over time and with practice. Same is true for American Sign Language. read more
Books aren't really ideal for learning a visual language like ASL. It is best to either learn in person (either by taking a class or practicing with native signers), or online. I recommend Rob Nielsen's website, Westwood ASL - Learn American Sign Language (ASL) online! He is Deaf and a native ASL speaker. read more
If you want to get started with sentence structure right away, try "The American Sign Language Phrase Book," by Lou Fant and Barbara Berstein Fant. The latest edition has a DVD with it. The pictures aren't the greatest, but it's great to learn basic ASL sentence structure. read more