You can be disciplined and aggressive.
First key is learn what pitches you can't handle, Casty drove me crazy b/c everyone knew he couldn't hit low breaking pitches yet he'd still chase them instead of spitting on them, if it's a strike, OK, you weren't going to hit it anyway, but most of the time it's a ball and you're then ahead in the count.
Second key is know you sweet spots, and with less than two strikes, look for pitches in those areas, because you'll hit tem at a much higher rate than chasing pitches you don't hit well. So if you're best at low fastballs, don't chase high fastballs and so on.
It's not rocket science, with the tools they have today, teaching hitters to both know their strength and weaknesses and then learn to see different pitches and how they move and whether they'll be strikes. With programable pitching machines, electronic strike zones and video, they should have hitters not just swing, but look at hundreds of pitches and get a feel for the strike zone and how pitches move.