That is a great question since you always want to stay top of mind with your customers and prospects. After making a great impression and having a productive meeting, you should always follow up with a thank you note. If you discussed a topic together and you can send along info on that subject, it can be a thoughtful reminder of your time together. It does not have to be related to your business, it can be a link to an art opening if they like cultural events, an article on a destination they will be traveling to for an upcoming vacation or even a white paper or article on a topic of interest. Some firms have clever leave-behinds such as desk accessories with a company logo, branded USB thumb drives or a magnet with their business card information for example, which can be useful and appreciated. Be creative but always start with a sincere thank you for their time and consideration. Good luck!