Well I haven't been able to completely conquer motion sickness but I've extended my tolerance to roughly an hour before I become ill.

In my earlier years it was pretty severe; I'd lose my lunch on even short car trips. Having access to a window helped, because the breeze counteracted the nausea.

I think part of the cause of motion sickness is the awareness that you are moving when--relative to all the objects around you--you are essentially still. If you can trick yourself into knowing that you are moving (step in place with your feet or have access to a breeze) it will help.

How to gain tolerance for reading or playing a game, though, I can't say. It's always made me ill, so I don't even try anymore.