Never include a sales pitch
"In a business setting, a thank-you note should never include a sales pitch disguised as part of the note. It is not OK to include, 'By the way, I will be in your neighborhood next week,' 'If you know anyone who can use my services ... ' or any of those tactics. Including a sales pitch dilutes the authenticity of 'thank you' and voids the feeling of reciprocity that would normally take place."