Trade routes usually follow what is available on the market screen in bulk.
Example. Reval-Riga area produces OIL, if you asses local prices it's usual in demand in Moscow area.
Don't sell to the same town each time, you'll do better spreading it out in the area.
Buying Spices/velvet in the South and selling In the North is very profitable.
Keep in mind that Caravans are not very repeatable, it takes time for city's to produce enough for a 'Bulk' sale to be available. It also takes time for Demand to go down so that it is profitable to sell to the same city again.
My advice is if you don't find a good available for caravan at a good price, just go to the next town. It's worth your time to hop around a bit before you commit to a 10day caravan.
Once I sold Velvet from Smolensk to Tula 3x in a row before the route disappeared. made 110,000 a trip
So it's also nice to 'Asses prices' in the market screen, though I'd save first to make sure the goods don't disappear.
Here's some examples
What From Price/total TO Profit
Fur Novgorod 40-4940 Kalanchak 20,000
Fur Reval 68-5830 Bratzlav 24,160
Oil Dorpat Fortress 37-4690 Moscow 9580
South East
Iron Kafa 17-4112 Novgorod 16,080
Salt Perekop 37-4472 Reval 11,200
South West
Raw Silk Bar Fort 530-20600 Narva 31,200
As far as making money, well If you have enough inventory space, just trade in High end items, velvet and spices and what not tell you have about 5-6000 then you can start the small caravans.
Theres a really good chance that a 20unit Caravan wont make money, its worth it to save up tell you can just do the 40unit caravans.
And don't buy the Trade Patent tell you have 55,000 so that you end up with at least 10k after you purchase it so you can continue trading.. (quest says it's 4,500 but it's really 45,000 yes it's been reported as a bug)
Good luck. I know I found it a much more challenging market then Warband