str77
When you start your trade route your big freighter becomes a tiny little freighter that shuttles between the freighter's original planet and the trading partner. You want your trade boosting starbases in the sector where the tiny freigher spends the most time.
I try to build freighters from my wealthiest planet and send it to wealthy planets of AIs that are least threatening especially minors. I hate to make my major opponents rich by trading with them but opinion varies on that.
There are some nice strategies at the Orion guide here http://www.orionsector.com/pages/galciv/downloads/The_Unofficial_GalCiv_Strategy_Guide_v1_0.pdf
One time I built hundreds of trade boosting starbases in three sectors along a trade route just to see what it could do to trades. There were so many starbases, I had to leave a little path so other trading partners could get to my planets. Each sector was more starbase than empty places. LOL.
The starbases can really bump trade but the constructor micromanagement can get a little old. Now I usually just send a freighter from my richest planet to the minors or a major in a bad situation. Then if I add two or three starbases in my home sector to add factories, I also build trade center modules in the home sector. It is easy to go overboard on constructor management so just have fun.
Rick