The best way to prepare for a shiatsu session is to bring loose, comfortable clothing. There's no oil involved so it's good to wear loose comfy pants, a T-shirt, kind of yoga clothing. And it's important not to eat for about a half an hour before so that you're comfortable laying down for about a 60- or 90-minute session.
It's great to think about the motivation for the session and if there's something that you want to focus on in terms of your health goals. So it could be an actual discomfort in your body or it could be something that's more long-term that you've been working on for a while. And the more clear on that you are, the more the practitioner can help you achieving that goal.
Often, a shiatsu practitioner will guide you and help you in your healing and well being goals. So the clearer you are on that, you can really get a lot more out of the session rather than just coming in and receiving whatever happens. So it becomes more of a conversation and you get to participate in your own health journey as well.