Also, the best way to make a lot of money with trade is to stay at peace with as many factions as possible. This presents a challenge for most of us, as we didn't buy Medieval: Total Peace, but there's something to be said for preying on the weak & keeping strong enough forces on your borders to keep your rivals at bay.

I am currently in a Danish campaign where I was making 6000 florins profit per year by 1150, and my only holdings were Denmark, Sweden, Norway, and Ireland. My opportunistic preying on my neighbors over the last 50 years has given me Great Britain, Sicily, Naples, Provence, and all regions bordering on the Baltic, and the only faction I am warring with right now is the People of Novgorod (who have, strangely, taken much of Asia Minor and the region around the Black Sea, but are staunch allies with the Byzantines... it's an interesting game).