How do I compliment a girl's appearance on a date?

We've gone out on 3 dates and I've really wanted to but never once complimented her appearance. I'm afraid of coming off as insincere because I like her but only found her 'cute' and not exactly pretty or beautiful. She also doesn't wear anything especially catchy that I can compliment on - she just looks plain and nice.

How do I compliment a girl's appearance on a date?
Post Opinion