In my experience, Shelties don't actually shed to much. They actually just hold their coat and hold it up it to big fat sheltie pants and then explode 2 or 3 times per year.
But if you regurarly brush your Sheltie, you will pull out buckets of fur that you didn't even know was there.
Bottom line....you should be brushing your shelties at least very two week. And bathing them at least every 3-4 months.
This is what will loosen the undercoat and encourage new growth.
And please, please....don't ever shave them!