Do you want a li'l somethin' to set your core ablaze? (and channel Karate Kid, obviously)
Try these puppies:
Keep your midsection tight as well as your butt (everyone forgets that bit). This will prevent your hips from twisting or swaying from side-to-side. Lock your elbows and keep most of your weight on your supporting, not sliding, arm.
If you can't keep your hips level, spread your feet apart to create a wider base. As you become more proficient, you can inch your feet closer together.
You also get the added benefit of challenging shoulder stabilization so it's great for overhead athletes or anyone who needs to work on the smaller muscles in the shoulder.
To make it harder, you can elevate your feet.
We use furniture sliders at SAPT, but if you don't have them towels on a hard surface work, as do magazines (totally used that before) on a carpet, or sliding your bare palm lightly across the floor will work if you're in a pinch.