Longest trade route is always shortest route trough all your planets with trade ports. 

So try not to build trade ports on all planets to extend length of your trade route

Building a trade port on every planet is usually a bad idea from the trade chain perspective. You have to manually snake it out. If you had a chain A-B-C-D-E, but there is a route from A-C-E, your chain will use that one. Thus if possible you'd want to make the chain A-B-D-E, so you don't build a trade port at C.

Now you can argue this is a bad way to do it, but its how it works.