We've tried various impression techniques (cardboard, sea foam) and always got vague results since our bones don't make a distinct impression. Using this wooden clamp has been by far the easiest and most repeatable method we've found for finding our sit bones. Simply adjust the width of the clamp until you feel like your sitting stable, with your sit bones on the narrowest part of the wooden clamp. Shift side to side and you can feel your sit bones fall off the edge of the wood clamp. You can really feel when your off by even 0.5 centimeters. This could also be done with a couple pieces of wood, but the clamp makes it easy to make small adjustments, and also happens to be just the right size and shape.