Travel to the best site as reasonably possible. This place should be away from the city lights, and should be upward from any source of air pollution. Always choose as high an elevation as possible; this will get you above some of the lights and pollution and will ensure that you aren't cursed by ground fog.
Try to use your telescope in the middle of a city, unless you only want to view very bright objects such as planets.
Observe on flat ground, away from buildings, walls or natural obstructions and away from any source of heat.
Try to observe any object when the light path is near any protrusion on the ground. Even extremely light winds can cause major air turbulence as they flow over the to of a building or wall.
Observe on the ground. If you try to observe on any structure, or even a sidewalk, any movement you make will vibrate into the telescope. Pavement and concrete will also store heat which will affect observing.
Observe from your house, deck, or apartment block (the worst).
Make sure that you have a dark, unobstructed view of the horizon, especially the southern horizon if you are in the Northern Hemisphere and visa versa.
Observe through a window.
Check the weather forecast. The best conditions will have still air, and obviously, a clear view of the sky. Note: It's not necessary that the sky be cloud-free. Often broken cloud conditions have excellent seeing.
Plan to observe in changing weather systems; these are always turbulent.
Observe near the horizon. You will be looking through lots of atmosphere, complete with turbulence, dust particles, and increased light pollution.
Observe the night's objects as they cross the meridian. The meridian is an imaginary line that runs from the Zenith,, due North-South. This is the point at which objects are highest in the sky. Observing at this time reduces all of the bad effects of the atmosphere.
View immediately after sunset. After the sun goes down, the Earth is still cooling, causing air turbulence. As the night goes on, not only will seeing improve, but air pollution and ground lights will often diminish. Some of the best observing time is often in the early morning hours.
Give your telescope's optics time to adjust to ambient temperature. Your mirror or lens is changing shape as it cools, resulting in a poor image. Allow longer cooling times for larger optics. If you are using an equatorial mount, use this time for polar alignment.
Use a star atlas or periodical magazine to find planets and other celestial objects.
Expect to point your telescope anywhere and expect to see interesting things.
Have a good selection of eyepieces and optical filters at your disposal. Experiment a lot.
Use anything except red light to see while observing. This will minimize effects to your night vision.
Try to go out with more experienced observers. Learn about their equipment and observing techniques.
Observe with both eyes open! This avoids fatigue at the eyepiece and prevents development of a lazy eye. If you find this too distracting, cover the non-used eye with your hand.
Expose your eye to anything except red light for 30 minutes prior to observing. This allows your pupils to expand to their maximum diameter.
Use averted vision on faint objects: The center of your eye is the least sensitive to low light levels. When viewing a faint object, don't look directly at it! Instead, look slightly to the side, and the object will appear brighter.
Look sideways into your eyepieces. Your eye is just another lens in the optical system: Keep it aligned.
Use eyepieces that give a magnification less than 1.4X the aperture of your telescope in cm. At less than this magnification, the exit pupil of the eyepiece will exceed the entrance pupil of the best eyes.