>>215192988
Stretch materials, the reason Roddenberry didn't want zips is because he had this big thing about how future clothes would be skin-tight, in the 60's spandex would have been pretty expensive for a TV budget so they had to have a hidden zip on the uniform shirt and on the boots to get them as tight as they wanted. When TNG came out, Spandex was affordable, which is why they were all wearing it until somebody managed to pry Gene out of the driving seat.

"Lost in Space" also had this idea that future people would wear really form-fitting clothes. They made it work by buying commercially available ski-pants and basically anchoring the actor's shirts in place with hidden snaps.