Monitoring and Evaluation
It is a good idea to note where enquires come from. This information can then be used to tell you which part of your marketing activity works best (and which is least effective).
You can list all your marketing activities (word of mouth, event, newsletter) and show how many enquires were generated by each activity and how much each activity cost. In this way you can work out all your marketing spending on a cost per enquiry basis. Future marketing spending can be focused on the areas that bring more results.