1. Make Sure it's Pretty
This is a no-brainer, but definitely one of the most defining factors for getting big bucks for your trade. Dents and scratches will undoubtedly lower its value, so always make sure you buff out what you can.
Also, any detailing done can increase the trade's value.
2. Have All Service Documentation Available
A record of all maintenance done to the vehicle over the years is a boon for the appraiser, as it allows them to make sure all upkeep has been performed so the trade can be given a higher pedestal on the used car market. It also means you'll probably get more for it.
3. Get Basic Repairs/Additions Done Prior
Having little things fixed here and there on the trade before taking it to be appraised will also bump its value. Repairs don't have to cost and arm and a leg, but if a trade recently had new tires put on or a fresh tune-up, the bottom line value will have a more favorable number.