Once you've closed a piece of business, the next step is to create a repeat customer. If your product is a one-time purchase, then the equivalent is to earn a recommendation (and, of course, you should also do that if your product is subject to repeat purchase, too).
In many cases, it's all about service and relationships.
The Rest of the Story...
When someone buys from you, they are agreeing to engage in a relationship. And so your job, whether you're in sales or marketing, is to use your customer's willingness to engage in a relationship to continue to uncover needs that you can fulfill.
Which brings us right back to research and planning.
Funny how that works, isn't it?