Increase the amount you sell per transaction.
Offer an upsell to every new customer to make sure they are aware of everything you have to offer.
Cross sell- offer them a product that is related to what they are already buying. Amazon does a great job at this.
Bundle Products- offer a discount for buying several similar products that go together and compliment each other.
Increase the frequency of transactions.
Communicate often with existing customers, let them know about all the new things you have to offer. Tell them about the new ways you are using an old product or service. Share the progress of your current projects.
Survey your customer list to find out what they need and find a solution for them.