Take personal responsibility for learning the language. Teachers are important, but ultimately it is the student who must study and commit to learning. This means that the student should be practicing on his or her own time.
Speak English as often as possible. Students have an advantage in that all they have to do is walk onto the street and they are immersed in English. They can also practice with their families and friends.
Make American friends. This may seem daunting and hard, but it is possible. Students can meet people through their children, in churches/synagogues/temples/etc, sports events, or other activities they enjoy.
Watch American TV; teachers can recommend appropriate programs. Soap operas are usually a good idea; the more dramatic the better. This is because they portray "real life" situations (at least in theory) and are often very dramatic. If possible, students can turn on English subtitles to help them learn vocab (especially if they are visual learners).
Attend town meetings/court cases, etc. These are useful for two reasons: students are not required to talk but can simply absorb, and they can develop a greater understanding of American culture.
Volunteer in their child's school.