At the very minimum you, should be visiting your key customers annually. The reality is that 80 percent of your revenue will come from 20 percent of your customers. Those are your key customers...get off your duff and go visit them. Is it going to cost money to visit them? Probably. If you spend $1,000 on airfare and travel to get a $10,000, $30,000 or $100,000 deal, would you say it was worth it? We have had clients that have added 15 percent of revenue by just visiting key accounts.
The best practice is to develop a system to reach out to existing customers. Pick a frequency, then pick the vehicle for communication. Put the process into your CRM system and then at the minimum, it can (should) remind you to do the activity. Even better, with the automation available in CRM systems, you might have the ability to automate most of the actions such as sending emails and scheduling calls and reminders. The only thing that can't be automated is actually scheduling the meeting with your customer since you need their input.
This is not rocket science. Spend 30 minutes and develop your first draft at staying in touch with customers and make sure that annual visits are one of the items.