here is where you stand, and what it's called.
(some parts of this may be UK specific, but I doubt it)
if your parents have a car then you want to drive it *sometimes* here is what you can do.
get your parents to add you as a named second driver on their policy
get your parents to add un-named second drivers to their policy (very expensive)
get comprehensive insurance on a real old banger so that it's really cheap, then the comprehensive insurance will let you drive Any car, (with owners permission) on a third party basis. (this would include your parents car)
if you have a car and you drive it each and every day then you are the main driver.
in that situation it's actually illegal to declare your parents as the main driver and you as a second driver, it's called fronting, and it's basically insurance fraud since you have to lie when filling in the forms saying that you won't be the principal driver.
http://www.fastquotes.co.uk/young-drivers.html
Named Driver on Parent's Insurance
It's possible to be a named driver on one of your parents insurance policies, but do be aware that if you have an accident their no claims bonus will be at risk. If your parents have a protected no claims bonus then this way of insuring young drivers is a good option, and enables the young driver to be on the road and gaining valuable experience. However, you won't start qualifying yourself for a no claims discount until you have insurance in your name, so in the long run this could actually cost more by delaying the start of your own no claims discount.
Insurance Mistakes
Do not be tempted to take out an insurance policy in an older person's name and have a young driver named on it. In the insurance industry this is known as fronting and mis-represents the risk on the policy. It is also tantamount to insurance fraud. Additionally, if the young driver ever needs to make a claim and is not specified as the main risk on the insurance policy, then the claim is liable for rejection by the insurance company.
My dad specifically wouldn't insure me as a second driver on his policy, and that actually worked out OK for me.
the insurance was pretty mental in the first year, (well over a grand) so there was a massive deposit and monthly payments in excess of £100. -and that was only third party!
but I had to ride that out.
I had a few crashes, but agreed to not get the insurance companies involved as they were minor, and it meant more to be able to hold onto a no claims bonus than it did to loose it, (cause I'd have paid more in the end).
within a couple of years I found that I was able to get insured on much bigger more valuable cars, comprehensively, (so essentially I could drive any car with the owners permission) for less than half the price of my first years 3rd party insurance on my own!
so best advice I think, (though it'll be a struggle)
talk to your parents, see what they can do to help you, you might find that you'll get a better deal going with the same insurance company as they do since multiple cars in one house often works out cheaper.
see if your parents will lend you money to get your own insurance rather than just asking to go on their policy.
assuming you're safe and sensible etc then when it gets to next year you'll have a years no claims bonus and you'll immediately notice the premium difference.
whilst your friends. well one day they are going to be too old to go on their parents policy. and then they are going to have to start paying big money cause they have no no-claims discounts...
I guess what I'm saying is it's better to suck it up now rather than delay the inevitable!
edit: - as well as fronting being illegal, if the insurance companies even suspect that you may be doing it they can, (and will) cancel your parents policy leaving them nu-insured.
http://money.guardian.co.uk/insurance_/motor/story/0,,1884126,00.html
D-Lew said:
It's probably just something they allow you to do to feel better about it, in actuality they aren't really losing money on you.
Fronting costs motor insurers as much as uninsured drivers - £600m a year or £30 for each insured motorist.
Oh well 
