But probably it's only because it's working as intended. Last session I tried a bunch of roller coaster sims, so maybe feeling sick just means it is working properly :)
For instance, when I tried DK1 for the first time, I was able to test many demos for like 45 minutes, and I didn't felt a lot of sickness. Meanwhile, a friend of mine held only 5 minutes.
This friend of mine (who owns a studio and he wanted to develop for virtual reality), started to train himself and his colleagues, using the Oculus every day till they felt sick. After 1 month, they were able to play Unreal Tournament with the DK1 for 15-20 minutes, and use normal demos for an hour or two.
I highly advise you to try this out too: you can just improve!