Have them prepare ahead of time. The test might be on Friday, but there's no reason they have to wait until Thursday evening to study. Have your child begin reading his material and reviewing for the test a few days before. It's much more effective than cramming it all into one evening.
Insist she find a quiet space to study. Studying in front of the television or at the dinner table is not going to cut it. They're not going to absorb much information, and anything they do manage to absorb will probably not be retained. Have her study in her bedroom or in another space in your house that is free from activity and noise. She will be able to concentrate better and retain more information this way. Get her into such a habit early on, and she'll probably carry it with her throughout college.
Review with them. A child can re-read and quiz himself on all the information he wants, but it tends to be much easier if someone goes over the information with him. Quiz your child on the subject, ask him questions, and offer to help in any way you can. The active participation of having to answer questions will aid him in remembering important information and hold him accountable for knowing the information.
Make learning fun. No child likes to study, but it doesn't have to be some horrible chore either. When you review with her, help her with any memorization through word association games. When you quiz her, reward her for a job well done. Make it an enjoyable experience so she is not left poring over books in her bedroom, banging her head against a wall.