Site selection
Terrain topography, sun exposure, wind direction. These are the general considerations one evaluates when selecting a camp and tent site. As the following comments will illustrate, there is no universally "best" criteria. Preferences and priorities will influence your choices, safety consideration aside.
In general winds have a prevailing direction, influenced by macro climate trends. For instance the Sierra in California have winds that blows from the west, caused by the effect of the earth's rotation and jet stream. But major weather events often cause the prevailing winds to come from other directions, even reversing the prevailing winds at times. There are also localized affects on wind. Typically wind in the mountains flow up from the valleys in day time, and down from the peaks at night. But terrain can alter this flow, for example the local terrain nearest your camp may be higher to the north, but in fact a deep canyon on the far side of that slope may be the regional low spot. So while you may expect the evening breeze to prevail from that local high point, the deeper canyon will cause the opposite, and draw the air from your camp, over the local high point, and down into the deep canyon beyond. These influences are complex enough that local conditions can sometimes perplex the experts. There is something to be said of local familiarity. We can go on and on about wind considerations; indeed it is an advanced topic. Become a student of your intended destinies, learn the specifics of these locations, including seasonal variations.
The whole point of attending to wind direction is twofold. On one hand is intentional exposure/protection from the wind for comfort. On the other hand there is often safety considerations. If a wind is strong it can damage your tent. Winds may also blow loose snow, causing it to settle in protected gullies and lee sides of slopes. These locations may harbor significant newly deposited snow which my present avalanche hazards until they become stable. Classic examples of bad camp site selection include camping on or below "snow pillows" (the places where wind blown snow settles below the leeward side of ridgelines) at the bottom of gullies, or run out zones beneath avalanche paths. Thus wind partially defines the topography of snow covered terrain. There is much more to say about snow/wind/temperature/terrain dynamics than can be stated in the scope of a forum post.
Sun exposure preferences are driven by personal preferences, as well as safety considerations. Most campers like to have early morning sun. But I am a late riser by nature, so prefer a site that provides the latest sun set possible. Safety considerations often override personal preferences. It is not safe to travel in steep terrain where sun warmed snow is apt to weaken and slide. Thus trekkers will start the day in the dark, so they can travel through these areas before the sun has the opportunity to warm and destabilize the snow. And even when avalanches are not the issue the best solution to travel often is getting it done early, while it is cold and the snow still firm enough to support your weight. There are few more miserable, avoidable circumstances than sinking up to your thighs as you slog through a snow pack in the mid day, when you could have walked on the top if it when it was still frozen solid hours earlier. These considerations can become strategic, influencing if you decide to camp on one side of a mountain pass or the other, to enhance safety as well as efficiency. It can even determine the general direction of the trek, which end of the intended trip will be the start/end point. When you get good at this you can even determine where first/last light will occur in the nearby vicinity, based on the current sunrise and setting points on the horizon, and the potential of nearby terrain and trees to block the sun from your camp site. But we are digressing into advanced skills again.
Topography will also drive camp site selection considerations. Some of these considerations are static. For example I prefer to camp on rock and soil versus snow, when given the option. I also prefer to camp higher than the nearby low points, since cold collects in low points. Camp above lakes, not lakeside.
Topographical features also affect safety considerations. As mentioned earlier, avoid camping in avalanche paths and the bottom of gullies. One should not place their tent under trees with snow loaded limbs - they may "bomb" you, damaging the tent and causing personal injury. It is not a good idea to camp on top of frozen lakes. This is self explanatory!
Site selection addresses the dynamics between these considerations. For example winds tend to accelerate as they pass over ridge lines and at the entrance to hanging valleys. Lee sides of mountain slopes tend to accumulate more snow than windward aspects. Camping amid a clump of trees or rocks may reduce exposure to wind and sun. Again this topic can digress into much detail.
Lastly how you prepare the tent site area has a big impact on your comfort. As mentioned previously I prefer camping on rocks; little to no preparation required! And the sun warms rocks, providing a good spot to dry out wet equipment. If one camps on snow they need to compact the area, so it remains level once occupied. This is done by stomping down the tent site. As Bill mentions tie the end of tent guy lines to rocks, sticks or pieces of equipment that can be buried under a foot of snow to form secure anchors. If winds my be a problem, use the snow to build barrier walls on the windward sides of the tent. DO NOT OPPERATE STOVES IN YOUR TENT! It is unsafe; the consequences are well documented - google "tent fire" and be elucidated. Instead build a cook stance for your stove. Make a wind screen from snow, and place your stove on a flat rock so it doesn't sink into the snow. All of this should indicate the necessity of bringing a good shovel.
Lastly travel safe. Many folks get into dangerous situations due to ignorance. Traveling up steep snow slopes in the warm day time, only to become stranded mid slope as cooling temperatures cause the soft snow to freeze solid and become dangerously slippery. Depending on your venue you may need crampons, ice axe and other technical gear. Safe travel entails many considerations, you need to school up on these, for most mishaps are travel related.
As you can see, snow camping entails lots of skills, knowledge, and strategy. A good starting point is the book: Mountaineering The Freedom of the Hills, published by the Mountaineers Books. But that is only a start. Depending on where you travel, you may need additional instruction, in part to acquire additional skills as well as enhance your safety. Try to travel with well experienced campers until you are confident in your abilities. Ask why they do what they do - about everything. I have been trekking for decades, yet still learn something new on a regular basis. Learning mountain lore is perpetual. We are only experts for a moment, then advents leave us behind, compelling us "back to school" to update our repertoire to the current state of the art.
Ed