Winter is an ideal time to do some pruning, especially when the temperatures are around 30 degrees.
Pruning is not recommended when the temperature is well below freezing, since the wood gets brittle at that time and will shatter when cut.
The advantage of winter pruning is that you can have a clear idea of what needs to be cut out and what can stay, at least when it comes to deciduous plants.
Also, the dormant plants will be easier to cut during winter.
Ornamental trees like Weeping Cherries, Flowering Dogwoods and Flowering Crabapples usually tend to send branches in different directions, so it is a good idea to prune the competing branches.
First decide how you want your plant to look, and start pruning.
Stick your head inside the tree and see what can be eliminated.
This is just like looking under the hood.
You will see a number of small branches which have not had any sunlight, and should best be removed.
Also, wherever you see two branches that are crossing paths, remove one of them.
Once the inside of the plant has been cleaned up, start shaping the outside.
This is very easy -- just picture how you want your plant to look, draw imaginary lines in your mind and cut off anything that is not a part of those these imaginary lines.
Mostly, your plant will have two kinds of growth -- terminal branches and lateral branches.
Every branch will have one terminal bud at the end, with a number of lateral branches along the sides.
When you cut the terminal bud, the plant sets multiple buds, thus making it look nice and full.
Trimming your plants will actually make them look nicer.