Discuss the homework assignment with the student you are helping. By discussing problem areas ahead of time, you can focus your assistance on those areas where the struggling student needs you most and address those issues before the student begins her homework. You may need to address them again later, but going over the problem areas ahead of time is an effective way of preparing the student.
Assist the student only when asked. Don't insinuate yourself into the situation unless the student looks as if he is having a hard time. Allow the student to do as much as he can on his own. If it appears the student is struggling, ask if he needs your help.
Work out an example when necessary. Don't do the actual work for the student. When she runs into a problem, use a similar problem to illustrate how it is done, then let the student use your example to work out the actual problem. Explain each example you give to help the student better understand the work she is having trouble with, but always let the student learn by doing.