Block the viewfinder
When you’re shooting long exposures with a strong ND filter, or you’re using the self-timer or firing the camera remotely, then it’s worth blocking the viewfinder and eliminating any light that may get inside your camera.
If you don’t, then there’s a good chance that stray light will enter the viewfinder and affect the exposure. In the worst examples – typically when you’re using a 10-stop ND filter – you can end up with a ghostly glow in the middle of the image. This can happen even when you’re shooting with Live View rather than the viewfinder.
High-end cameras have a built-in eyepiece blind, but most DSLRs have a viewfinder cover that can be attached to the camera strap and swapped with the viewfinder eye cup when required.
Holding your hand close to the viewfinder or draping a cleaning cloth over the top of the camera works just as well though.