Quote Originally Posted by weegee
Not sure, but I think upgrading the trading posts to the next levels might also increase trade. I forgot to track it on my last upgrade, but I'll see if I can note how much trade I had before and after.
Some posted the following information somewhere in these boards (forgot where, as I tend to gather info into docs for myself ) :

(from an Org post)
Trading post: 100% (baseline - you get to trade)
Merchant: 120%
Merchant Guild: 140%
Master Merchant: 160%

It had already been said, but try to trade from provinces with 2-3 resources first, establish a network of boats (beware of bad weather/aggressive factions like the sicilians who can bring your trade income to almost 0 by sinking one boat).

Hope it helps