Anytime you change your home's architecture, it's important to keep any new elements in balance with the old elements. This is especially true of dormers. If you are adding four dormers on one side, make sure they're spaced evenly and that both sides of your house will appear congruent. If you're adding a single dormer, position it in the middle of the exterior, unless other visual elements exist to balance out a single dormer placed elsewhere.
As you plan, keep both the slope of your roof and the type of roof you have in mind. You typically want to have your dormer roof mimic the style of your existing roof to maintain the exterior's visual appeal.
If your home's exterior is a little bland, use your dormer to add some detail. Add a bit of historic appeal to your home by installing windows with multiple panes in your dormers. If your home already has historic appeal, play up the trim. Add gingerbread trim to the dormers of a Victorian home, with the same trim elsewhere. If you're shooting for a more modern look but still want the detail, place a window box with some bold, bright flowers outside each dormer window. You can purchase window boxes in a variety of styles and materials from your local home improvement store.
Once you've added your dormer, don't waste the attic space you've created. Turn your attic into an extra bedroom or a bedroom and bath combination. Use dormers and niches in your attic to create cozy places in your new room. With a little bit of work, you can create a window bench in a dormer and cover it with pillows for a quiet reading spot, or build a desk in a dormer to create a sunny home office work space.