Actually, does anyone know the mechanics about when trade routes reset? It certainly does not seem to be every turn. I've had situations where nothing resets until after anarchy/new trade with a new AI - basically what happens is after somehow getting cut off from some AI for like a turn, I never get trade routes back with them in the normal course of things. Or alternately, I can already have open borders with many civs, filling out my routes - and then I get OB with some new guy (but no trade routes with them). But then when one of the original civs cuts off for whatever reason I go back to having domestic (not opening up new routes - anything should be better than domestic). For anyone who might speculate it's cause I don't have actual connections or something: I know it's not that, nor do I think it is a lack of total foreign cities somehow "available" due to the large map sizes I play. I suppose next time I come across an example I'll post it but if anyone knows more about the straight mechanics/code I'd be interested. Thanks in advance.
Also, about buildings like Harbor/customs house - this may not be always perfect advice but I'd say that trying to maximize trade routes is nearly pointless in BtS (ie don't build customs houses unless it's a really obvious need - it's very misleading because you'd think they are worth it but they are not). This is due to the fact that (in BtS) base commerce rarely gets higher than about 2 per route - you already get a significant bonus from population and so the building adds a very meager bonus. Invest your hammers elsewhere.