If you're keeping your car outside, you'll need to get a cover that is waterproof and breathable to prevent the paintwork deteriorating. You'll need to clean the car every time you put the cover on it, too. And some car covers aren't rated for high winds (or will need an additional locking strap). You'll also need to keep an eye on on the cover and disperse any heavy rain (or snow) that builds up on it. Even if you get a breathable cover, it needs to be removed in periods of high humidity or long spells of rain to allow the moisture to evaporate (this also applies to heavy snow, which won't allow the cover to breathe). Finally, the cover itself will need cleaning so check that you can either wash the cover while it's the car or that you have access to a suitably sized washing machine.