For many people, the days after a first date are a crucial juncture for their potential relationship. During this time, it's traditionally important to give your date some indication about whether you enjoyed the meeting, and whether you'd like to see her again. One appropriate way of doing this is to say something sweet or affectionate to your date to demonstrate your continued enthusiasm for seeing her.
A Small Fact She Brought Up
When speaking to your date later on, mention a piece of information that she originally told you on the first date, but probably wouldn't have expected you to remember. This demonstrates that you were listening to her carefully before, and that you've been thinking about her since your first meeting. This especially works if you also can turn it into a compliment. For example: "You always move so gracefully -- I wonder if that's something you picked up from those dance lessons you mentioned?."
Think about Detail
Often it's tempting simply to compliment a person's general appearance. Although there's nothing wrong with this, a compliment could be made extra thoughtful by noticing a specific detail instead. For example, consider the accessories (such as earrings, necklaces and rings) your date was wearing when you met. If any of them stood out in your memory, consider mentioning it to her. For example: "I keep thinking how lovely you looked wearing that pearl necklace last week."
What Reminds You of Her?
If you've been happily remembering your date since you met, tell her what causes this. For instance: "Every time I walk past the park I think about how great it was to see you there." This works particularly well because it demonstrates that you're already developing fond memories of your time together, and therefore it makes sense that you'd like to see her again.
If You're Not Interested in a Second Date
Even if you enjoyed some parts of the initial meeting, you might still feel that a second date isn't a particularly good idea. However, it's important to communicate this in a way that will minimize your date's feelings of rejection. It's possible to be straightforward with your date without being offensive. For example "I really enjoyed meeting you, but I just didn't feel that magic click. It'd be great to stay in touch, though". Or if you prefer, you could use a white lie to avoid hurting her feelings, such as "It felt so nice laughing and having a good time with you, but it just kept reminding me of how me and my ex used to be. To be honest, I don't think I'm ready enough to move on yet."