You don't want to harass clients, but you do want to be in constant communication. If you aren't, another company is! I find the best way to communicate with clients is email. Depending on your industry, you may want to do this monthly, quarterly or yearly. A quick "health check" to see if you can help with anything. Ask them if they have anything coming down the pipeline. Tell customers about new products, service offerings, promotions, etc.
Be sure to keep customers informed on your successes. For example, if you have closed a large sale in the food manufacturing industry, be sure to tell your other customers in the same industry! People like to learn what is being done in their industry. Perhaps a customer didn't know you were able to sell or service a specific item. By telling them what you have done in their industry, it might start a dialogue about a project they are looking at.
I also find electronic newsletters are a good way to keep in touch. It is a way of providing info on new products, services, etc. You can include links to white papers, videos, other websites, upcoming trade shows you are participating in, etc. You might tailor the newsletter by industry, geography, existing vs new clients, etc. Publishing these on a regular basis is key - whether that is bi-weekly, monthly or quarterly.
Depending on your industry, you may want to also consider social media - Facebook, Twitter, LinkedIN and Instagram are all excellent ways to keep your customers informed. It allows you to share information quickly and with a large group of people.
At the end of the day, regular communication with our clients will help with retention. Let your clients know that you value them and hopefully they will continue to buy from you!