Eyecups. That, and diopters. Glasses are too uncomfortable to wear, except when driving, so I tend to use diopters for my photograping purposes.
The factory supplied F3/F4/F5/F6 "eyecup" is just a rubber covering for the regular eyepiece ring. Commonly lost. DK-4 and DK-2 (and DK-19) have a metal ring that holds the eyecup in place over the eyepiece ring. The F2 generation eyecup has threads allowing it to take a regular eyepiece, which helps hold the eyecup in place on an F2 (or an F - tend to use it on the F, whereas the F2AS has a DK-4 eyecup). The Nikomat eyecups are the worst. Keep falling off the eyepiece ring and disappearing, never to be seen again. The one sold for the F was almost as good as the F2 version, except that there were no threads, so you couldn't use it with a diopter.
-J