Currently not open, but it is never too early to start preparing. Here is Claude Nicollier’s advice
(might be even useful for any position in the space domain or life):
Carry out activities that require careful preparation, concentration, and management of risks, such as aviation with aerobatics and instrument flying qualification, deep sea diving, mountain climbing…
Stay healthy and fit: run, play tennis, use your bike, but always stay safety conscious
Learn English, Russian and Chinese. Travel to countries where these languages are spoken and practice with locals
Learn as much as possible in a great variety of fields: technology, science, biology, physiology. Be good in your field, but never forget to acquire knowledge in other disciplines.
Keep a positive and pleasant attitude in every situation. Stay calm and cool even when things go bad!
Practice communication skills: give lectures, interviews, learn the art of skilful use of social networks, without overdoing it!