I actually think it is best to steer clear of compliments on her physical attractiveness until you are already in a relationship, preferably with a sexual aspect to it. Pretty women get told they are pretty all the time, and sometimes it can just get old or feel like an effort to get into their pants. Compliments about how much you enjoy their company or what you admire about their personality are much more rare and meaningful. However, once the two of you have a thing going on or have had sex at least once, go nuts with the compliments about her physical appearance, and phrase it in whatever way feels comfortable. She'll want to be reminded that you find her attractive once you have something established. Also, it can be sexual or non-sexual after that point too. Presumably if you are already having sex then making a more sexualized comment wouldn't be unwelcome either.