The other thing is to watch the fellow skiers. In skiing, the downhill skier is the person who has the right of way so everyone else needs to yield to that person and make their adjustments accordingly. If someone is coming down the hill and they fall, you want to try to avoid them and if possible see if they need assistance.
Some of the common things to keep in mind while on the slopes are watching your speed, being aware of what others are doing, if you're merging from one hill to another, make sure you look uphill and make sure you're not coming in and cutting someone off. You also want to observe closed signs and use your safety bars.
Another important principle is to know your limits..the terrain is constantly changing throughout the day and so is your ability as you get tired. If someone comes across a situation where they see skies crossed in the air that's definitely saying there's a problem ahead and you need to slow down if not even stop to make sure you're not coming into a situation and making something that's bad even worse.