One of my favorite "Long-Axis" Drills is what we refer to as the Side-Aligned Balance Drill.
Basically, you begin at the "T" in the pool, in a streamlined balanced position, not moving or touching the wall. You want to be pressed in the water in the chest area (on your stomach or back depending on which long-axis stroke you wish to drill), with a flat back (suck your belly button up and back), and your forehead (or back of your head if you're on your back) pressed "in" the water.
Finding Your Balance Point
At the "Go", begin kicking, and take a single stroke and remain on your side with one arm forward and the other at your side—your balance point now becomes your armpit. Make sure you're pressing on that point of your body and trying to shape your body as narrow as possible—by cutting water on both sides of your body.
Kick about 10 to 12 beats and then take three strokes (hand hits) and hold your opposite arm in a front, streamlined position while your other arm rests at your side. Kick 10 to 12 more times and repeat.
Getting Your Body in Balance
The longer you work this drill, the more you'll become aware of your body's balanced relationship with the wave line and how it cuts through this surface area. You can also concentrate on your arm position and the scull action used to generate your strokes and how the timing of your hips rotation can become involved in the forward momentum of your body.
Remember, your body cuts through the water fastest when you're on your side. Try to stay as long as you can without pausing too long at the top of your stroke. Long-axis swimming is all about rotating a balanced and aligned body around your head/neck axis.
Find out more about Colorado State University Swimming and Diving at their website.