I have always done fine with trading in old ones. Besides all the other suggestions about being super clean inside and out and touched up, here is a trick that helps to get a better deal.
I go to a dealer and tell him right up front that I am not buying a car today, but want to know his best deal. Then, I mention a dealer that is nearby where I am going next to see what they will do for me. I do fib a bit and tell them this is the first dealer I have been to, so give it his best shot before I go to the other dealer.
He will sharpen his pencil if he thinks he is in a competition for your business. When he comes back, simply tell him you think he can do better than that, so you will talk to him after you see the other dealer up the road. Normally, they will lower the price even before you leave.
Then, go to the other dealer and tell him the same story, that he is the first dealer you have visited, etc...as above. Then, decide who you want to go back to and dicker with to get your best price. I can guarantee (my so was a car salesman for a while) you they will give you a better deal if in direct competition. The last time I traded, I used the Honda against the up-the-street Toyota to get the best price.
If the price is close, then after leaving for a while (so they think you are checking with someone else), then return and say you wanted to tell them you think you will trade with the other dealer, because you could get more money for your trade in at that dealer...They will again bargain with you for several hundred dollars in most cases.
Works for me, give it a try.