If you're serious about asking her out, face-to-face is the only way to go. Not the only way, but definetly the most effective.
Most girls are so struck by the action of you working up the bravery to ask them out that they will have a very hard time saying no.
Of course, many serious relationships start over the phone or IM. One of my best friends has been dating a girl for eleven months now whom he asked out over the phone.
But I'm a dying breed of "hopeless romantic", and I think it should always be done face-to-face.
Just my opinion/advice.
-Z
"Nothing takes the taste out of peanut butter quite like unrequited love."
-Charlie Brown