What you can do to make hypnosis safer and more fun for you:
learn self-hypnosis
Trance is trance, wether you're being hypnotized by someone else, by a video or audio-file or just by yourself, it's still teaching you what it's like to follow suggestions, to go deeper or to wake up. Practicing these skills will make you better at going into trance in the future, which many hypnotists appreciate. Not everyone likes a tough nut to crack.
You can even give yourself suggestions that will improve your confidence and safety as a subject, for example, suggestions that remind you of your safeword, that help you to speak up if you need something (like going to the bathroom), even when hypnotised, or that help you recognise and refuse unwelcome suggestions. I sometimes do some self-trancing when I want to make sure there are no lingering effects after I've used a hypnosis file online.
learn more about inductions and suggestions
Once you know something about the technique, you can more easily recognize suggestions, and therefore decide whether you want to accept them or not, and why. When you know something about inductions, you can figure out which ones will work for you and which ones will not, and which ones you just don't like. Once you know something about what hypno play could entail, you can make an informed decision about what you want from that, and you can more effectively communicate with your partner about your wishes and your limits.
ask your hypnotist to tell you about their experience and where they learned to do hypnosis
Get to know them. Not just whether they have any experience at all, but you also want to know whether they are just a hypnotherapist who wants to have some fun in their free time, or a mind-control dom who wants to teach you new things, or a spiritual person who wants to help you grow. Make sure you want the same things. Get to know each other. And don't hesitate to conclude that this is not the hypnotist for you. I know, it's hard to find a kinky hypnotist. But really, you're better off not having play, than having things go awry.
ask the hypnotist to include safety mechanism in their suggestions
The hypnotist can remind you of your safeword and your ability to say it anytime, no matter how deeply hypnotised, or reassure you that hey never want you to feel pressured to do anything. They can also include limits to their suggestions: only when I ask you, only when you feel safe, until the end of this session. Such limits could prevent unwanted or lingering effects.
If someone tries to hypnotise you before you've done the getting to know each other part, or before they've negotiated with you about what they should and shouldn't do with you, don't go into trance. Bite your tingue, squeeze your hand, whatever you need to stop yourself from going into trance. This is a really unpleasant situation, and a totally dick move. Walk away. Don't answer their message. Or get angry, that's totally justified.
Here's some more advice pulled from my class for people who like to be hypnotised that I teach at Charmed:
Hypno is a skill you can practice and get better at like playing the piano. It's unhelpful to get frustrated because you can't do it perfectly from the get go. Here's some things you can practice as a beginner:
Just going in trance and noticing what that feels like for you
Using your safeword/waking yourself up
Resisting/adjusting suggestions
My advice is to take time with a practice partner to work on these things first without getting into hot hypno fantasies. Much like starting out with rope, practice the basics first.
Here are some ways you can practice active cooperation during a hypno scene to help you get better at following suggestions and feeling hypnotised:
When the hypnotist says "you're feeling soo… <example>" try it. Imagine what that feels like. Use your internal dialogue to help hypnotise you. Just repeat some of the hypnotist's words in your head or out loud "Soo deep…" Maybe your inner dialogue can make it into a mantra. Some people find this really hot and that can be a motivator to do it more, and to feel the effects more.
Practice responding and adjusting a trigger. Triggers are inside your head, they are yours to give to other people to use (or not). You decide what the trigger makes you do or feel and who gets to use it.
Play along. It’s ok to fake it till you make it, because that’ll build neural pathways to help suggestions work better in the future.
Practice saying no. Sometimes you don't want to a do a thing right now. A trusted play partner is a very good person to practice this important skill with. It's also good practice for them to say "ok, thank you for stopping when you need to. Let's do something else." Maybe it's helpful to take turns practicing this.
Figure out, through practice, which modalities work for you. Do you react strongly to sexy hypnotist voices? You might have a preference for auditory stuff. Do you dream vivid images when in trance? You might be a visually oriented person. Or do you have trouble with visuals? That's valid. It's helpful to know these things about yourself so you can tell hypnotists about it.
Install safeties and contingencies in your head. If you're not yet comfortable doing this through self-hypnosis, you can enlist the help of a hypnotist friend. "I will only go into trance when I feel safe and comfortable doing so" or "Whenever I'm uncomfortable or in need of assistance, I will easily and automatically use my safeword" or "whenever a suggestion would make me uncomfortable or move me to a headspace I don't want to go, I'll find it easy to just ignore the suggestion".
If you're a person who spontaneously forgets what happened in trance, you can learn to remember it through practice. If you're a person who has never experienced hypnotic amnesia and it's something you'd like to try, there are ways to practice and figure out how to make it work for you.
The Unconscious Mind is just a metaphor, a way to help us understand how the human mind works. It's not a separate entity in your brain, your unconscious decisions/actions are your decisions/actions. Understanding your subconscious reactions helps you understand yourself, and you can work together with the unconscious part of you to keep you safe and improve yourself.
Because hypnosis is happening inside your mind, you are the one most able to protect and advocate for yourself. Sometimes you are the only person who knows that something is off, so you need to tell your partner about it, or they simply do not know. So practice communicating your needs during a kink scene, constant communication is really helpful. A sigh of "this feels so good…" can be just as useful to your partner as "wait, hold on, I need something".
In the same way you experiment and figure out your preferences in inductions and modalities, you should figure out what you need as aftercare after a hypno scene. Having your needs met will help you be a better bottom and a better play partner. Don't let anyone tell you you're "topping from the bottom" when you're just talking about your needs and your limits.