Abbie,
The best advice I can give you is to combine a few differant variations of the previous posts.
Firstly, look at Buy and sell, Autotrader and basically any magazine that allows private advertisers and try to get the average price for your car. Make sure you are comparing like with like (ie same year, engine type, body style and similar mileage as your vehicle). Don't bother looking at dealer websites and dealer price lists. If a dealer is advertising a vehicle for sale at €20,000, you can be sure that he's not going to offer someone with an identical vehicle €20,000 on a trade in. There's no profit for them, so it's logical to assume that the offer with be several thousand less.
Secondly, take a few digital photos of you car. Open the golden pages and ring around as many Toyota dealers as you are prepared to travel to. Speak to the salesman and ask for his email address. Send him the pics of your car and ask for price. Whatever price he gives you ask how much he is prepared to better it by. Never tell him, (and he will ask) how much you want for it, tell him it's his job to be competitive. I would then say that you will think about it and hang up. Write each price down and when you've called them all decide if you are prepared to part exchange for the money the best two or three dealers quoted. Starting with the third worst price, call them back and advise them that you have a better price and ask them will they better it? Continue with all three dealers along this line until you sense that they will go no further. At the end of all that you should have a pretty decent price for your car without driving around dealers and negotiating on their territory. Don't be afraid to play dealers against each other, as long as you don't lie about a price you received you'll be okay.
When you using this method with VW last month the differance between the best and worst prices I got was €5,500!