At 18, I've found...
Add both your parents as drivers. It brought down potential Mini Cooper S insurance from £3000 to £2,200 pa. I suspect age is actually irrelevant for this. I spoke to a bloke the other day who drives a Focus ST and he said adding his girl friend on the quote actually reduced it. They're both 27.
Also, if you did Pass Plus put it down in the license type options. Again, knocked off loads on higher insurance groups cars.
What's interesting though is what to put as your occupation if you're living at uni away, but don't take your car with you. I know UNI-LIVING AWAY is an option but when you select that do they think you'd be taking your car with you or leaving it at home.