Cascara Sagrada Bark

I purchase a bag of Cascara Sagrada dried bark, cut and sifted. I have used it, but is there anything I can do for the bitterness, and is there a certain amount I should use?

There is no way to remove the bitterness because in part the medicinal effect of the bark is due to the herb's bitter principles. All you can do is to mask the bitterness, typically with sweeteners such as honey or sugar or by adding fruit juice.
According to John Lust's "The Herb Book" (available from Richters), a typical dosage regimen is 1-2 cups of tea a day, taken before meals or on an empty stomach. The tea is made by steeping 1 teaspoon of bark in 1 cup of water for one hour.
Another option is to make a tincture. Lust recommends taking "40-60 drops with water, morning and evening, as required." This method may be less bitter to take.