No matter what instrument you use, dark, clear skies are essential for deep-sky astronomy. Some galaxies are only slightly brighter than the normal background skyglow, so you need all the contrast you can muster from your instru-ment and your eyes. Before you begin observing, give your eyes 15 or 20 minutes to become dark-adapted. Then use a red-filtered flashlight to study star charts without ruining your night vision.
If you live in or around the city, you can effectively boost the contrast by using light pollution reduction filters on your telescope. These are especially useful for nebulas, many of which are normally invisible from cities.
You can also gain a little contrast with the technique of averted vision. By one side of a deep-sky object, you avoid a "low sensitivity" patch on your retina.
For especially low-contrast objects, use averted vision and the "jiggle" method. Simply give the telescope tube a light tap when you think a nebula is in the field. The eye can detect contrast differences more easily if the image moves slightly.
Another hint is to keep both eyes open when looking through the eyepiece. Squinting shut the unused eye causes strain that can affect the active eye's vision. If you are distracted by the passive eye's close-up view of the telescope tube, simply cover the eye with your hand or an eyepatch, To completely eliminate visual distractions, you can drape your head in a dark hood. Your breath may fog the eye-piece in cold weather, so keep the back end of the hood, open.
Finally, concrete, asphalt, and other hard surfaces absorb heat during the day and slowly reradiate it at night, creating local air turbulence. Always try to observe from a grassy area.