Only compliment her once, if you compliment her over and over she is going to get uncomfortable. Avoid generic compliments, she will know that it was just a lame compliment and not feel like it even counts.
Good ways to compliment a girl is to notice something specific. Like, does she have full lips, green eyes, or high cheek bones? Make a compliment that is specific to her. Notice what her style is. Does she dress classy or trendy? A woman wants to feel special and if you take the time to notice something specific to her, you will do just that!