Check out edmunds.com. They'll give you the dealer's cost, the sticker price, and the price that people in your area are paying. Since the Yaris is a new line, I doubt you'll get it at dealer's cost. However, it's not a hybrid or something in crazy demand, so I think you can try to get something between the dealer's cost and the sticker price.
As for your trade-in, check out bluebook.com. It'll give you a rough idea of what you can get for your car. Also, if your car has internal problems (engine, etc.), don't volunteer that information, as they don't usually do more than turn the car on and look at the exterior when deciding what to give you. My '92 Celica needed $2700 of engine & other work. It's Blue Book value was "N/A."
But I got $300 for it!
I just bought a Toyota a few months ago (got a Prius
), and it looks like some of the Toyota dealers are now trying to sell their own private service plan, besides the extended warranty stuff that Toyota sells. My dealer tried to sell me a service plan that was only good if I brought the car to his dealership. Unless you plan on going to the same dealer every time you need service, don't go for that sort of thing.
Also, some dealerships have websites & have slightly lower prices online. See if that's true for your dealership. If it is, print out the lower price & bring it with you.
Anyway, lots of stuff. The best two things I can say are 1). Look at bluebook.com to figure out what your trade-in is worth, and 2). Find the dealer's cost on edmunds.com & try to get as close to that price as possible.
Good luck!