One of the advantages of pruning during the winter is that you can see much better what needs to be cut out and what should stay. At least that's true with deciduous plants. The other advantage is that the plants are dormant, and won't mind you doing a little work on them.
Ornamental trees should pruned to remove competing branches. Weeping Cherries, Flowering Dogwoods, Flowering Crabapples etc. have a tendency to send branches in many different directions. It is your job to decide how you want the plant to look, and then start pruning to achieve that look.
But first stick your head inside the tree and see what you can eliminate from there. This is like looking under the hood, and when you do you'll see a lot of small branches that have been starved of sunlight, that certainly don't add anything to the plant, they are just there, and should be cut out. Any branch that is growing toward the center of the tree where it will get little sunlight should be cut out. Where there are two branches that are crossing, one of them should be eliminated. Once you get the inside of the plant cleaned up, you can start shaping the outside.
Shaping the outside is actually quite easy. Just picture how you want the plant to look, and picture imaginary lines of the finished outline of the plant. Cut off anything that is outside of these imaginary lines. It is also important to cut the tips of branches that have not yet reached these imaginary lines in order to force the plant to fill out.
For the most part plants have two kinds of growth. Terminal branches and lateral branches. Each branch has one terminal bud at the very end, and many lateral branches along the sides. The terminal buds grow in an outward direction away from the plant. Left uncut they just keep growing in the same direction, and the plant grows tall and very thin. That's why the trees in the woods are so thin and not very attractive.
When you cut a branch on a plant, the plant sets new buds just below where you cut. When you remove the terminal bud the plant will set multiple buds, this is how you make a plant nice and full. Don't be afraid to trim your plants, they will be much nicer because of it. The more you trim them, the fuller they become.
Lots of people have a real problem with this. They just can't bring themselves to prune. Especially when it comes to plants like Japanese Red Maples. It kills them to even think about pruning a plant like this. Just do it! You'll have a beautiful plant because of it.
Look at the plant objectively. If you see a branch that looks like it's growing to far in the wrong direction, cut it. If you make a mistake it will grow back. Not pruning is the only mistake you can make. I hope this helps and doesn't get you in trouble with your significant other. Many a family feud has started over pruning.