I started my son last year (2010-2011) at age two. I was asking this same question. Whet method should I use? I learned at a very young age with a harness, so I considered that method. But the more I talked to instructors, the more I wanted something different.
Then I ran across a Hookease ski trainer and found what I wanted to use! The Hookease device is a clamp on the back side of both skis, then a hook attached to the end of your ski poles. You insert the hook from your ski poles into the hole in the clamps on their skis. Now you can control there speed, position of skis, and the edge they are on.
I used it in conjunction with an edgie wedgie, which they (Launch Pad Gear) made a much better version of. They called their edgie wedgie a Wedgease. It is super quick and easy to use. I'm amazed nobody has came up with this sooner.
The Hookease allows you to have full control of them without putting them in the back seat. They are offering discounts right now of 20% off. Take a peak at launchpadgear.com.
I have been really impressed and want everybody to have the same experience as I did.
Feel free and send me a message if you have any questions.