Generally I take the approach that anybody who misses an appointment will have done so without any malicious intent. For this reason I don't levy a missed appointment charge but I will phone or write to a failed client explaining that I don't charge and invite them to re-book, which usually works.
Rather than worrying about chasing missed appointments I would rather consider ways of ensuring appointments are kept. A text message to clients on the morning of their appointment is great for this purpose. The following link might be of interest for this.
http://www.esendex.com/uk/Default.aspx