When I was doing the flight school, the first 20 hours were early morning or late evening to avoid turbulence. At around 30 hours, I was flying light planes very low over fields and forests in the middle of the day, I had motion sickness so bad that I was barely able to ride my motorcycle back home. I asked the instructors about this and they said something in the lines of "we all had it, it is going away with time".

The next year I never had motion sickness when I was in control of the plane, but if I had a copilot when we were switching control I still had some motion sickness in bad conditions. Several years later I almost never have any problems, unless I spend too much time looking at the maps and not outside of the cockpit. It goes away with time, it seems. I never tried sailing in the past few years to check if I can get seasick, but I noticed I am almost unaffected by car sickness that I had a lot when I was a kid.

One way to partially train for this was doing a couple of in-flight maneuvers that provoke sickness to almost anyone, including some seasoned pilots: inducing close to zero-g conditions, either by doing short slingshot maneuvers (point at 20-30 degrees nose-up and then push the stick keeping 0.5g, then 0.3g, then 0.2g) or doing it faster while extending the flaps to full, the sensation is very bad if someone does it to you, but it can be even enjoyable if you doing yourself flying alone. I practiced the slingshot part for a few weeks, this is what it took me to get to ~ 0.2g without feeling sick.