The Car Insurance Comparison Sites
You'll need to use insurance comparison sites if you want to reduce your insurance premiums. Firstly (in this order) check Compare The Market, Google Insurance Compare, Confused and GoCompare. If you still want further results, try MoneySuperMarket and QuoteZone, although results from these two can vary.
Next, visit the sites that aren't listed on the comparison insurance websites - these are currently Direct Line, Aviva and Zurich. Sometimes they are cheaper as they don't pay a commission to the comparison sites which can sometimes be as much as 30%.
Finally, look for offers. You can get store vouchers and gadgets/products from some websites if you buy insurance from them. Car insurance offers are normally emblazoned on their front pages.