Observe Away from Buildings
Observe away from buildings, pavement or large objects that absorb heat by day and release it at night. When these objects release heat at night, they create air currents (heat mirages) which degrade image quality in your telescope. This is why observing from a terrace or top of a building is not a good idea. The best locations are open, grass covered areas.
Window Telescope Observing
For a similar reason, observing through an open window is also a bad idea, especially if there is a pronounced difference between the air temperature in your house and the outdoor temperature. Since air always flows from a region of warmer temperature to a region of cooler temperature, you instantly create a nasty air current when you open the window. This seriously degrades the image.
Close Window for Observing
If you must observe through a window during cold weather, leave the window closed, but be aware that the window glass is now acting as a lens in your optical system and your optical system is only as good as its weakest link, which is now your window. Since the window is acting as a lens, you will also discover that best image quality will be obtained by aiming the scope directly through the window, rather than at an angle. Pointing the scope up or down, rather than straight through the window, will produce serious optical distortion.
Observing from a Deck
For the above reasons and more, observing from a deck is also a bad idea. Not only does such a site put you too close to a building, it also provides a less than stable observing platform. Every step you or someone takes will instantly produce a vibration in the eyepiece of your telescope and the higher the magnification, the worse the problem. If you have no other option, fine, but a telescope needs to be on the ground for best results.
Adjust Your Eyes
Allow your eyes to become dark-adapted before trying to observe faint deep-sky objects. This takes time - typically 30 minutes under truly dark conditions. Unfortunately, it only takes seconds to ruin your dark-adapted eyes by looking into a bright light. Since red light is easier on dark-adapted eyes, astronomers therefore use red light to work around a telescope or read star maps. You can either buy a flashlight with a red lens or make your own by coating the lens with several layers of red nail polish.
Observe with Averted Vision
Use what astronomers call "averted vision". Simply put, this means looking out of the corner of your eye (where your eye is more light sensitive), rather than the center of your eye. In other words, don't stare directly at a faint object when trying to see it - glance at it from the side of your eye. It can mean the difference between seeing a difficult object and not seeing it.