The best way to prepare for an interview is to look presentable and profsssional, as you would for any interview. Be prepared to answer questions such as what you feel you could contribute to the spa if you were to be hired and why you are interested in working there. It would also be wise to share any relevant experience you may have from the past.
My interview was very sales oriented. If you have sales experience, great, be ready to talk about it. If not, make something on your resume relate to sales or customer service somehow. It was mostly just to get to know your personality.