After just about any meeting, successful or note, it never hurts to follow up with a thank you note. It shows you respect your client's time and even if your product was a good fit this time, your client knows you're a courteous, professional colleague he'd like to do business with again.
Email or snail mail? You know best; think about the client's company culture. Usually email is fine, especially if you're thanking for something informal like time.
Your tone is also something only you can gauge. When in doubt, more formal is better. You don't have to be cold or stuffy but do be respectful.
Here are a few examples you can customize to your own situation.
It was a pleasure meeting you yesterday. Your ideas on new approaches to sales in our region were insightful and a great help to me. Thank you for making time to meet with me and for sharing your thoughts.
Dear Dr. Price,
Thank you for seeing me yesterday. I appreciated the chance to discuss our new products with you. Your opinions are always helpful and interesting. I hope the latest product will fit your needs.
It's always a pleasure to see you!
It was a pleasure to finally get a chance to meet you today. Your research project sounds very interesting and offers a compelling value proposition to to the mobile phone carriers. As discussed, my team has done extensive work in the type of data collection that your research requires.
Please let me know if I can provide you with more details about how we would work on the project.
Look forward to speaking with you soon.
Thank you for your time today. Jack and I very much appreciated your input on our new business development strategy. I know that you have a number of demands on your schedule right now so the fact that you took as much time as you did to guide us is even more welcome.
We will be sure to circle back with you when the plan is finalized in order for you to provide your stamp of approval as well as any final comments.