Again thorough training in keeping a good lookout and developing a high level of spatial awareness helps to mitigate the risk.
And compared to powered flying, not having a zillion dials and electronic gizmos in the cockpit means you spend more time looking out. Through a very large clear canopy instead of tiny windows. Plus the speeds in gliding are relatively low compared to powered flying. And gliders circle a lot, which means aspect changes which make them more conspicous than somebody on a straight and level track.