Yes, whenever you see uranium as a tradable resource then start to build a freight unit. Even if your city loses the resource (which doesn't stay very long) while your freight is in transit, you'll still get the massive bonus. Use airports to skip between continents to get the big bonuses. And don't forget that it's often more worthwhile to send a freight across a really short sea to a city very close than to send it half the way around the world to a city on the same continent. Many people keep records of which of their cities supply and demand which resources on databases and the like, but don't be ashamed of a raggy bit of paper. But be aware that the supplied and demanded resources change during the game so check every so often and exploit them. You don't want to lose all the valuable trade routes without realising it and replacing them. I loathe it, but in Civ embracing capitalism is definitely the way ahead.