[JoshTriplett]

> Yeah, I once played with someone whose strategy was "Never trade".

That's the right strategy for strong play. You should never trade property for mere money. If you absolutely have to, trade property for property if the trade gives you a monopoly, or if (in a more-than-two-player game) doing so gives you a net increase in the number of groups for which you hold at least one property (so you can block anyone from getting a monopoly in those groups without trading with you).

It's not an especially fun strategy, either for you or the people you're playing with.