OK, I've fussed over my car insurance quite a bit over the last few years and have managed to find out quite a few bits of information which may help when you get insured.
1. Get insurance in your own name as soon as you can afford it. I managed to persuade an insurance company to give me two years NCD after driving on my mum's policy though this can't always be done. Driving on someone else's policy also implies that THEY drive the car most of the time. It often isn't found out who drives a car most on a policy but always be aware of this technicality.
2. Convictions must be declared and may not make a difference until renewal, in which case if the premium goes up too much, change insurers! Different companies consider different convictions differently. For example, my own company won't charge anything extra for a red-light offence, though an SP30 (3 points for speeding in a 30mph zone) will cost me an extra £100 for renewal. It costs nothing to ask just out of interest and the company shouldn't act on your enquiry as you haven't stated that you have that conviction. If you suspect getting points from your driving, ask when you get the quote.
3. Always declare your modifications. It secures your car against the insurance company ignoring your claim if you have an accident. You should declare everything, even a steering wheel change can increase the premium. Remember it's not just the mods that make the car go faster they worry about, it's also the things that make the car look more attractive to thieves too. Remember that if you decide to line your interior with gold-plating!
The key part of this for 205 owners is that GTis are easily recognised by thieves by their bolder styling and alloy wheels etc. If you put GTi identification on an XS, such as the plastic bodykit, then it's more likely to get nicked. Then when the police find it with the GTi bodykit on, your insurers are unlikely to be very friendly about it. My insurers would allow me to put the plastic mouldings on the side, but adding the plastic wheelarches would cost me an extra £90! It is very surprising what they pick up on and you can't expect to remove all your mods if you have an accident. (If you're in belief you'll never have an accident because you're such a good driver, find out what happened to my last car) Alloy wheels should be declared for an XS as they were optional extras at the dealer NOT standard equipment like with the GTis. You may be able to get away with thinking they're standard, but some insurers may still prove difficult.
Car engine modifications and other mechanical alterations are obvious things to inform them about. Again, they can pick up on small things. My insurers allow me a full performance exhaust system with no premium increase. I can also do anything to the transmission (like a different gearbox or quickshift conversion). They won't allow me engine modifications such as a filter or changing the carb, gas-flowed head etc. So check for mods that are not allowed. Always bear in mind that insurance companies hate paying out claims and if they have an easy way of dodging it they will. To find out that your car isn't what you told them it was is like a gift to them.
4. Looking around for insurance. Remember the obvious things like: