1) Shop around and buy online: Figures show that many people simply renew their current policies without shopping around. The internet makes it easy to compare prices from different insurers, so why not take advantage of this? Plus, you'll usually get a discount of 10% or more just for buying your policy online.
2) Policy type: do you really need a comprehensive policy with all the extras? Going for a third party fire & theft policy can reduce your premiums hugely, and is definitely worth considering if your car isn't an expensive model.
3) No claims discounts: Nearly all policies feature a discount that increases for every year you don't make a claim. The higher the discount available, the more you could save. Also look at insurers offering a 'no claims bonus for life' feature, where your current discount level can be fixed forever, even if you have to make a claim somewhere down the line.
4) Excess: The excess on a policy is the amount of a claim you have to pay before the insurer pays the rest. Choosing to have a higher than standard excess level will usually mean lower premiums.
5) Security: Fitting your vehicle with an alarm, immobiliser, or other security devices can lead to premium reductions. Parking you car off-road, for example on a driveway or in a garage, will also mean a cheaper policy.
6) Pay annually: Many insurers charge you interest for the privilege of paying in monthly installments. Pay annually if you can afford it to avoid this, or look for one of the companies who don't charge extra for monthly payment.
7) Mileage: The more mileage you run up every year, the more your insurance will cost. Even if you can't reduce your mileage, make sure you're not overestimating how much you actually do drive, and give your insurer an accurate figure.
8) Drivers: The more drivers you have on your policy, the more it will cost. Reduce the number of people insured to drive your car to the minimum possible, and try to get the policy in the name of a driver with the lowest risk profile. For example, if a car is driven by both a man and a woman, insuring it in the woman's name will often result in a cheaper quote.