Start by choosing the right car. Supercharged rockets inevitably carry high premiums, as do imported and modified vehicles. Next, consider the driver. Earning a 'no claims bonus' and avoiding convictions lowers premiums, as does limiting your mileage.
The next stage is to shop around. Premiums vary considerably between companies, so establish a 'baseline' - then beat it. Existing providers send renewal quotes, but never accept these without looking for comparisons. Get several quotes and include companies specialising in specific demographics, e.g. young drivers.
Premiums can be reduced by increasing the excess, improving vehicle security, paying online, and paying annually. The latter avoids monthly interest charges. The trick is to remember that increased risk equals increased premiums. Buying sensible cars, driving responsibly, and protecting the vehicle therefore minimises your expenditure.