Just like developing a streamlined body position requires performing specific drills, developing swim-specific strength requires specific exercises.  There's no better way to develop this strength and muscle memory than working with tubing as a part of your dry-land training.

Training with tubing is easy to do on days when you're traveling or just can't make it to the pool.  It can be done up to three times per week for most athletes.

If you don't have swim tubing, get some.  It's essential.  90% of athletes use medium resistance.

Here's a link to purchase yours.

Phases of the Pull

Although the normal pull segment of the swim stroke is one fluid motion, it's helpful to divide the pull segment into phases to describe and train each portion more effectively. 

1. High-Elbow Catch – The catch is the first third of the pull and begins immediately after extension of the arm in front of the head.  This is the most critical portion of the stroke because it's the most unnatural and initiates all of the strokes propulsive power.  The upper arm (shoulder to elbow) is held high in the water.  The shoulder juts forward.  The upper arm twists inward so that the elbow points up and out.  The lower arm (hand and forearm) arcs downward as one unit while the upper arm and elbow remains high in the water.  The catch creates a paddle where the lower arm is positioned in front of the head and ready to press rearward to propel the body forward.  
    
2. Diagonal – Following the catch, the diagonal is the middle third of pull is where the bulk of the power production happens.  The upper arm begins to angle down in the water as the elbow continues to point out and is bent at about 45 degrees.  The upper arm presses rearward collapsing the armpit.  The lower arm presses rearward as one unit with the elbow and hand maintaining the same vertical alignment. 
    
3. Finish –  The finish phase is the final third of the pull and occurs just before the hand exits the water near the hip.  The elbow extends and the hand flicks rearward as the arm begins it's out-of-water recovery.