There is no proven way to recruit good non-executive directors but there are certain ways to gain access to the right kinds of people.
Business colleagues who have made use of non execs in their own work may be able to make personal recommendations. These won't necessarily be people they have worked directly with but it could be associates of associates - basically, classic networking.
Once the preserve of the larger, richer companies, the use of search consultants is on the increase for medium and smaller companies. A consultant finds and matches up non executives with a client company - for a fee.
Services such as the Non-Executive Directors Association (NEDA) can also help smaller firms find a non exec right for them.