Make sure you have the correct IPD set (Inter-pupil distance). If you don't know what yours is you can check with these free Android or iOS apps.
Chewing Gum - Multiple people report this helps while playing
Use Teleport instead of smooth motion, most games have a an option for this
Start with games that you don't move at all inside the game. Like "BAIT" is a free game. You sit in one spot, like your favorite chair, and play from there.
Start slowly, only a few minutes at a time. As soon as you feel any illness starting, STOP PLAYING!
Most games have settings to help with motion sickness. Set them at the most comfortable level they have. When it comes to moving inside the game use teleport and slow turns, to begin with. Some games will allow you to automatically skip things like climbing and will place you after the climb.
Have a fan blowing lightly from in front of you. This keeps you cool (heat will cause the illness) as well as always having a signal to you as to your location in the room at all times. Lowering the temperature of the room prior to playing helps.
Stay out of games that have a lot of movement like a Rollercoaster or flying through the air.
When you need to turn in the game, turn your body itself instead of using the joysticks or hand controllers.
Use the "stationary" settings instead of the room settings. It will keep you in one limited area and warn you if you're moving too much.
Stay HYDRATED. Drink only water (no alcohol) drink plenty before during and after playing any VR games.
Eat a little plain, blah food before playing. Having a little something in your stomach will help. NOT spicy foods! An empty stomach can trigger illness. White bread Toast with a little butter or noodles works great.
You can try different store-bought remedies for motion sickness. There are many different kinds of Pills, as well as a wrap for your wrists with pressure points that can relieve the sickness. And some people suggest eating "ginger" but it has to be the REAL thing, NOT like a "ginger ale" in the soda section of your store.
The fit of the headset and the clarity of the viewing can affect you. You don't want a headset that is very tight to your head, the pressure to the forehead can cause headaches and other illnesses.
The screen viewing needs to be as clear as possible. Adjust the lens spacing inside the headset to fit your eye spacings. If you wear glasses, you can buy for around $50 your prescription lenses that just pop on and off the in headset lenses. Never have to worry about wearing your glasses again while playing.
This process can take weeks to months to develop a buildup against motion sickness. Best of luck and happy playing VR games to you!
Whatever you do, do not listen to those who say "power through it" That will only lead to illness, throwing up, severe headaches, and dizziness that can last for hours afterward, and your body rejecting playing the moment you start to play.