Joss, in addition to all the good advice here I'd recommend checking out a book called "Don't Get Taken Every Time" by Sutton. There's some great chapters on financing, as well as negotiating the purchase.
One suggestion from his book: if you're buying new, the down payment must be at least equal to that first hit of depreciation, which can be north of 30% on some models, IIRC. Otherwise, as Chuck and others have noted, you end up owning more on the car than it's worth--"under water".
Sutton also has some good formulas for figuring out a) how much money does "x" payment a month give you, and b) for figuring out how much that payment costs you in interest. Good stuff, and if you've got the dough but want to build up good credit, why not take a one or two year term loan, pay down as much as you need to, and pay the loan off in one or two years versus 4--you'll save a ton of interest and hopefully get the benefit of being a good payer. If you get an idea of what the car might be worth at the end of its usable (to you) life, that may also help you figure our how much to put down. Edmunds, both the little book and the website, is a great resource for this kind of data.
A possible alternate strategy, also mentioned above: buy a slightly used but still factory warranted car. That way the depreciation hit has already been absorbed, but you still have some factory level coverage. Balance it against the cost of an extended warranty(that worked for me--a Toyota Platinum extended warranty more than paid for itself, but that may not be true of all makers), which you may need to buy any way if you want to keep the car beyond the normal warranty.
Also, look out for terms on Zero per cent financing; it's not available to everyone(credit reasons), and if you have to pay list price in order to get it, measure it against the possible savings from a discount off list and the added cost of interest payments.