I'm on AS and am averaging just under 3.20 BB's per game. My only adjustment is the Strike Frequency slider which I've only dropped one click (to 4).
A lot of things factor in BB's. I mean, so many. But first...
Absolutely disregard the tip about always taking the first pitch (I'm not singling anyone out. I read this all the time on OS). The CPU is programmed to recognize this pattern. If you always take, you will almost always begin to see first pitch strikes. And once you begin to see it, you need to break it...by absolutely demolishing the pitch (which will lower the pitch and overall confidence, which will improve BB's).
Pitch Speed is important. If it's so fast that you are constantly chasing...especially on 3 balls counts in order to protect...and always checking your swing, that means it's set too fast. You need to be able to see the ball in order to take the bad ones.
Foul balls are essential in prolonging pitch counts and forcing the pitcher to miss. If you are not fouling off in the neighborhood of 20 per game (though, realistically it should be higher), that means every time you actually commit to a swing, you're actually putting the ball in play (which will probably make your subconscious tell you to feel bad for swinging the stick!). You cannot walk if the ball is put in play. And to go with that, you cannot walk if you swing (I know, I know. In one breath I'm telling you to swing, and in the other I'm telling you not too. LOL! I hope I make sense here
). Shoot for taking in the neighborhood of 45% of all pitches. Though, not every game. Just as an average.
The pitch history is a beautiful reference tool. CPU pitchers resort to patterns. In "Behind Count" in pitcher history, if you sort it by batter handedness, you'll be stunned at the type of repetition you see. Often times, you'll find there that 4 out of the last 6 "Behind Count" pitches to a RHB were (for example) low changeups out of the zone. So when you get in that situation again, look for it. Heck, even use the Guess Pitch and Guess Location options if you need to in order to isolate those pitches to make it easier for you to take.
Also, it's no secret that Veteran level will allow more strikes than on All-Star and above. I'm not recommending you change any levels, but you need to keep this in mind. This is why you will very often see Veteran players drop the CPU pitching sliders. Whether it works or not, I cannot say, because I don't play on that level.