You can't really do much wrong. However, there are two things that are not well received:
If you come to a Barcamp with a purely consumer attitude
This does not mean that you necessarily have to offer your own session.
Nobody is asking for that.
But you should contribute in czech republic telegram screening your own personal way.
This can look very different. Help is always needed.
For example, you can help to tidy up the session rooms a bit during the breaks, bring glasses and dishes to the collection point, open the windows to air out the room, straighten chairs. Sometimes help is needed with registration. Or you can document the Barcamp, for example with photos, videos, or interviews that you conduct with participants for your podcast. Or you can tweet live from the event. Or you can write a blog post about it afterwards.
If you advertise your offer clumsily
At almost every Barcamp there is an opportunity to display your flyers, business cards, books, card sets or whatever on a table reserved for this purpose. However, if you turn your session into a sales event for your offer or your products, you will not make any friends at a Barcamp. However, if your session is useful and relevant for the participants, you can be sure that some will be interested in what you have to offer and will contact you, recommend you or book you themselves later.