Nice ways to remind someone you are thinking about her include sending her a short message, joke or picture, telling her about something she might like, or offering her a small gift. These actions can make her feel present in your mind even when she is not with you.
Be encouraging and positive with your jokes and pictures, and make them specific to the person you are engaging. Ask questions about her that allow her to be expansive with her responses instead of giving single word answers. Listen to what she has to say to you in response. Tell her how you feel about her.