Averted vision
Looking slightly to one side of a faint object, so its light falls on the most sensitive part of your eye.
The centre of your vision is packed with colour-sensing cones, which are poor in the dark. The rods, which see faint light, are spread around it. So glance about 10–15° to the side — towards your nose — and a faint galaxy or nebula can brighten or appear from nothing. Experienced observers gain about a magnitude this way.
At the eyepiece
Combine averted vision with gently tapping the telescope tube: the eye notices moving faint things better than still ones.