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.
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.
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.
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.
Allow you 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.
Make sure your finder scope is aligned properly before you take your telescope out under the night sky. The finder scope is the little spotting scope on top of the main telescope and it helps you center objects in your telescope. For a discussion on how to do this, see my article at Telescope FAQs.
Always start observing with the low power eyepiece. This is the eyepiece marked with the largest number, not the smallest. It is much easier to find an object at low power and images are brighter and sharper as well. In fact, you will use your low and medium power eyepieces much more than your highest power eyepiece.
Keep the magnification down. Too much magnification is, perhaps, the single biggest beginner mistake with a telescope. Too much magnification yields a fuzzy, very dark image. There are a great many variables involved as to how much magnification to use, but when you begin to lose image detail or when images are not as satisfying to your eye, back off on magnification. Be especially careful when using barlow lenses - it's very easy to overdo magnification when using this accessory.
Begin your observing with an easy object such as the moon or a bright planet - basically things which are easy to see and find. Trying to find faint objects such as galaxies and nebulae takes practice - it is as much a matter of technique as it is equipment. Go slowly. As you gain experience and confidence, move on to more challenging objects.
If you have a GOTO model, use alignment stars that are widely separated in different areas of the sky and that are lower to the horizon. This gives your computer a better fix on your location than when using alignment stars that are close together and overhead. Doesn't hurt to learn the names of a few brighter stars, either, as an accuracy check for your computer system.
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.
Keep your expectations reasonable. Compared to an observatory telescope, your backyard telescope is minuscule. It will never deliver the image quality or detail you see in magazines or on television. Nor is your eye a camera - it cannot collect light over a long period of time like a camera and produce the beautiful color images you see in photographs. Those are done with filters and computer enhancement.
On the other hand, your telescope is delivering an image directly to you and to you, alone. Astronomy is now your own personal voyage of discovery into the depths of the universe and your telescope is your spacecraft. Even a small telescope or binocular will give you a glimpse deep into the depths of interstellar space, but, in the end, it is the experience as much as it is the view. The glory of it all is that we can do it from our own backyard. Enjoy.