This commonly held belief is untrue. Car insurance is calculated quite scientifically based on a series of factors which determine the risk you pose. This includes the make and model of the car, the number of miles driven and your driving record. You cannot simply negotiate a better deal. One insurer may offer a lower quote than another but this is because of the different method they use to assess your risk.
There are, however, steps that can be taken that may lower your car insurance premiums. For example, you could install a black box in your car to track your driving habits or add a safe driver as a second named driver. Either of these measures might reduce the cost of your insurance.