I tried and tried to figure out "loosen if it doesn't downshift", etc, but it just wasn't working for me (I'd later learn why ).

From what I learned last night --

http://www.bikeforums.net/showthread.php?p=5512365

I'd try using the barrel adjusters to set the cable tension just enough to get rid of any slack.

If it's too tight, it'll jump gears; if it's too loose, it'll do all kinds of weird stuff (really, it's bad; maybe I should've taken some video of mine).

What worked for me was making sure that the cable would start pulling immediately when the shift lever was moved -- but to not be so trigger-happy that it pulled too much.

I had accidentally taken virtually all the tension out of the cables, so doing the quarter-turn-at-a-time with the barrel didn't have any effect. Both the front & rear shift cables ran along the downtube, so they were easy to examine. If I flicked the cable with my fingers, it would just flop lazily. The derailleur finally started behaving when I took out all the slack -- but if I made it too taut, it would start misbehaving again.

So, it ended up working best just a few turns on the tight side from being slack. Once the cable had enough tension, it would make a distinct musical pitch as I plucked it. I didn't keep track of the actual concert pitch it produced, but I began using the pitch as confirmation that I was actually adjusting cable tension. It worked much better than trying to eyeball the derailleur's motion & alignment, and I was still clueless when it came to watching how the chain shifted across the cogs.

Of course, this relies on the shifter's index function, too. If it's good, it'll work exactly as designed. It just won't work if it's damaged somehow, though.