Sometimes parents get caught up in the anxiety of this time and stress for their children. This could be more stressful to your child and they may think you don't have complete faith in them. Instead try to provide a calm environment and encourage them in a positive way. Also providing "study snacks" and nutritional meals can help them to study better.
You can help your child to study effectively by making sure they take study breaks so they don't burn out. But don't let it get out of hand where the break sessions are longer than the study sessions.
Not all children are able to study from a book. Because they learn differently they need different studying techniques. It's easier to know how to help them when you figure out what their study style is.
Parents should also realise that their children won't be good at everything. Some are better at maths and others are better at English. If you can identify in which areas they excel, then you can concentrate more on the ones they struggle with.
Bribing your kid to do well isn't a good idea. It's common to hear this phrase around exam time: "If you do well you'll get [insert expensive reward here]". Threatening also shouldn't be done. How can a child perform at his best when he is so anxious about the hiding awaiting him at home if he doesn't. Instead encourage them to do their best and help them set their own personal goal to achieve. You can't expect him to go from a D to an A overnight.
How are you helping your child during the exams?