First call the right pitch location, then sell it to the batter and ump
As much as you'd like to put all the onus on the pitcher to throw strikes, a lot of the responsibility rests on the shoulders (and the hips) of the catcher.
Let the batter call the pitch
The strike zone is batter specific. So check where the batter stands - up in the box? down in the box? away? or crowding the plate? Does it change after the first pitch? Did the batter move back indicating he thinks he's too close for the pitcher's fastball and is expecting another. Or did he move up in the box, waiting on the pitcher's changeup? It can tell the catcher exactly what pitch to call.
But not to extremes
Lateral positioning is not reliable. A smart batter standing away from the plate may be protecting the inside corner - knowing he has good extension to the outside. Or he could be waiting on an inside pitch. A batter crowding the plate could be trying to force the pitcher to throw outside. Or he could be waiting for an inside pitch - his strength.
Let the previous swing call it
A batter who uppercuts may have poor perspective on low outside pitches. Or he can be pitched up and in - causing him to miss or pop out. A batter with a predictable, level swing may drawn into an unnatural swing on a low and away pitch.
Frame it to get the call
This is the most controversial section of this page. As one umpire pointed out to us: any umpire worth his weight knows whether it is a strike or ball as soon as it hits the front of the plate - framing doesn't help. Some experienced players would say that if the ump detects body movement mid-pitch, you won't sell it (contrary to the prior plate-crossing statement). But then the body-shift method below was from a seasoned pro coach and catching instructor. We'll let you decide what works for you - in your league, with your umps.
Not all strikes are swings and misses.
Often to get called strikes, you may want to frame the pitch. However, very important,
frame only the close ones or the umpire will ignore your efforts. In fact, some umpires frown on framing so know who's making the calls and what he prefers. (Below the images are other important points about framing.)

Glove framing... Login to see it In-Motion.

Body shift... Login to see it In-Motion.
NOTE: Rest of framing discussion relies on seeing and understanding the two QuickTime clips. Both clips are longer than normal, showing a variety of pitch locations and/or comparing framing to shifting.
Please login or join.