Rule no. 1, any list of proven buyers will outperform any list of non-buyers.
There are general guidelines:
- Recency: When was the last time they have made a purchase? [MOST IMPORTANT]
- Frequency: How many and how often they bought a particular item?
- Unit of sale: How much they have spent would be a good indication of their willingness and openness to buy.
Read more: http://www.thegaryhalbertletter.com/newsletter-archives.htm