It's the background (clouds, trees) wizzing past that tends to make you dizzy because your eyes flick all over the place trying to lock on to something. I've always suggested to beginners to fix their eyes on the largest part of the model.

The other thing of course is the balance mechanism in our ears but that's simply a matter of getting used to it and the dizziness from that will stop. But that's kind of related to how fast you turn around so the longer the lines the better. Longer lines means a bigger engine and model but they're much easier to fly anyway. With the small Cox type planes you might be doing a lap every 2 seconds but with the bigger model and lines a lap will take up to about 6 seconds. That's a fairly slow rate so you get used to it much faster.

But training the balance mechanism really only works for the rate you get used to turning while flying. Get on a sideshow ride and you'll still get dizzy