Into the Glades
Once you venture off groomed trails and into any glades you need to know you are increasing the possibility of being hurt, i.e. running into a tree. Trees are not bendable slalom poles. There is no give to a tree and a head-on, or even an arm smack, can cause serious injury. However, that being said, and hopefully understood without the need for practice, here are a number of precautions to take to make tree skiing as safe as possible.
Trees don't Move - Ski Where Your Ability Allows
East, West, or in between, do not randomly ski into a stand of trees. Most ski areas have marked gladed areas available that are rated for standard ability levels. However, there is little or no real tree skiing for abilities less than high intermediate. Remember - grooming machines don't fit in the forest - so, you need to be able to ski in the various conditions you will find there. You will find powder, crud, iced over ruts, etc. - whatever Mother Nature puts up on a given day.
For the most part, tree skiing means quick, short turns - many of them. If the glade is marked double black or higher, expect sudden or protracted areas combining the trees with steep verticals, bumps, cliffs, and other fun obstacles. Be sure you are ready to ski there.
Ski With Friends and Stay in Sight
Ideally, ski with two friends, one to stay with someone injured and one to go for help. It doesn't take more than maybe 10 or 20 feet to lose sight of a person in the woods. While most marked glades lead downhill to open groomed terrain that is not always the case - especially in areas marked for higher ability. If you lose sight of your partners, call out until you can regroup again. Actually, a whistle carries a lot farther than yelling, but try to use it only when you really get that creepy, all alone feeling.
Be Bound By Resort Boundaries
In a ski resort setting the area boundaries are generally marked by tree signs, snow fences or ropes. Don't randomly ski past the boundary markers because that area is not routinely patrolled.
Watch the Time
A good general rule is not to enter the woods after 3:30 PM, because if you get lost, getting found quickly, after dark, is problematic.
Backpacks are Great Friends
Carry whatever you want, but at least bring water, some energy snacks, a compass, and a trail map for orienting.
Look Between the Trees
The lower the elevation, the closer together the trees are, and that can be very close together. Don't focus on the trees, look into the spaces between the trees - the body tends to initially follow the eye.
In the East and Midwest
I shouldn't have to say always wear a helmet and goggles, but I will. In the East and Midwest, the glades are lower and thicker, so don't ski with your pole straps on as they can easily get caught on branches. Be careful of the depth of the snow cover. If there's too little snow it won't cover evergreen root systems which can ensnare a ski tip with very bad results. Also, be extra careful skiing "slides" - bare rock, but, inviting verticals, cleared by mud slides.
In the West
Again, I shouldn't have to say always wear a helmet and goggles. The big issue tree skiing in the deep snow country is tree wells. Tree wells are areas of very loose uncompressed snow that form a hole or depression around the base of a tree. Evergreen trees, especially, have large and low hanging branches that can hold a lot of snow. This prohibits snow from filling in and consolidating around the base of the tree. These holes are hidden from view by the tree's low hanging branches and a head first fall into a tree well can be dangerous.
Recognizing the dangers of a tree well and taking preventive measures are very important. The risk of falling into a tree well is completely avoidable, so assume all trees have a hazardous tree well. Do not ski close to the tree and always ski with a buddy.
The Northwest Avalanche Institute, Mt. Baker Ski Area, Crystal Mountain, and Dr. Robert Cadman have put together Tree Well and Deep Snow Safety for anyone skiing trees anywhere in deep snow.