Prepayment energy meters

Looking for comment from energy/money experts about prepayment energy meters. 

Things I need answering are:

How can households cut costs if they have a prepayment meter?

How much cheaper will your energy be if you switch from a prepayment meter to a postpay (standard) meter and how easy is it to do this?

Can households save money by topping up their meter before price hikes take effect?

What should you do if you can't afford to top up your energy meter?

Thanks