First, let me say that I have never personally witnessed any major issues of concern on Tri-met (other than the occasional loud people).
However, ever since living in London a few years ago, I have wondered whether the answer to Portland's transit conundrum lies in TFL's system. For those of you who haven't been, TFL runs their fares almost completely electronically. They use a scanner system much like access cards in buildings to record entries and exits from the system.
My thought is that perhaps a card system that had entry scanners on all MAX doors and bus front doors, and could be quickly verified by a handheld wireless scanner used by fare checkers, would massively increase the speed of fare checkers and allow them to cover ground quickly. That has to be coupled with zero-tolerance enforcement system wide. This means that after one ticket, an exclusion and an arrest for theft of service is appropriate!
In the meantime, how about some Tri-met and police joint stings. Pull a train into a station. Anyone on it without a valid fare is removed from the train and ticketed by the line of Tri-met checkers waiting for them. See how many warrant arrests you get from that.
A few of these kinds of checks at random on the system would get attention real, real fast.