The first step in highlighting hair is choosing your hair color(s) wisely--this choice will have a big impact on how subtle or dramatic your highlights turn out. To achieve a subtle look, stick to colors that are only two or three shades lighter than your natural color. If you're a brunette and dying for platinum blonde highlights, you'll have to dye your entire head of hair lighter color before you start. However, if you like a bolder look and don't care if it looks natural (because it won't), you can go for light blonde or a daring red regardless of your color.
Another way to achieve a natural look is to choose a few different shades of highlights to give your color even more depth. If you hair were highlighted by the sun, it wouldn't happen in two uniform shades. You could go for two different lighter shades, or you may want to add a lowlight along with a highlight. The lowlight should be two or three shades darker than your natural color.
When you get highlights, you can go for either a full head or a half head. A half head often looks most natural because the sun wouldn't be able to lighten the hair underneath. However, if you don't want to see darker hair when you put your hair up, you may want a full head. You'll also want to pay attention to the width of your highlights. Fine ones look the most natural, while too-wide highlights end up looking striped and streaky. Varying the width between super fine, fine and medium will look natural.
If you part your hair or style it a certain way every day, keep that in mind as you (or your stylist) decide where to place the highlights. You'll want to have the lighter pieces in spots that'll make the most impact. Highlights around your face, for example, add brightness and definition as they frame it. If you have bangs, they should get highlights, too.