The Italian economist Pereto figured out that 80% of your revenue came from 20% of your customers. Maybe you have a 90/10 or 70/30 rule - but it's the same principal.
Relatively speaking, very few of your customers provide an awful lot of your revenue. These people are your lifeblood - you should lavish attention on them. Invest your marketing budget to keep your best customers buying from you - give them no reason to buy elsewhere and keep them loyal.
Once you know who they are and where they live, you can prospect for more people like them.