I've just purchased an ERGObaby carrier, and need your advice about making it work better for my daughter. She finds that the black straps dig into her thighs and cause her discomfort.

She may be sitting in too much of a pouch. The back of the carrier should be up as high as her shoulders or just under her armpits. Pulling up on the back of the carrier and tightening the shoulder straps will give more length in the body and bring the webbing straps higher up.