Pretty much what everyone else said - hit the internet and the comparison websites. I know its a bit of a slog, although if you've done it once and copied out all the essential details on a bit of paper, it makes it much easier for all the other sites. We've just saved £120 from our car insurance by just using Compare the market (Ok, we have a meerkat on the way), and we've used Compare.com in the past.
Car milage should be on a form somewhere, and its important to get it right - all the info you give has to be correct, so its worth making sure its right, rather than guessing.
You can go on the cashback sites, but that can be even more of a hassle. Greenclouds advice sounds really good, although I really couldn't be bothered to do much ringing around (my current insurance company said that their online quote was the best the could do - £100 more than the one we went for).
There is some good advice here:
http://www.moneysavingexpert.com/car-insurance/
Its not just car insurance - we saved loads when we looked at our last quote for house insurance by going elsewhere (discount helped). the crazy thing is that they rely on inertia, which is OK for a couple of years until it gets silly, and then they give you a much cheaper rate as a new customer, and then rinse and repeat. Just give us a decent quote and there would be far less churn.