You want to keep in touch but you don’t want to be a pest. Where is the line between the two? How much is too much? How often is too often?
Because each prospect is different, there is no rule to follow. Your prospect will tell you (if you ask) what his or her preference is. But you can be on the safe side if you make contact monthly or every other month, depending on how close they are to being ready.
Frequency, however, is less important than relevancy. When you keep in touch, to ensure that they are open to receiving your message (whether via email, voice mail, fax or in person), be sure to do it with something relevant to that particular prospect.
For example, if you know they frequently attend trade shows and you have a great article about making the most out of trade shows, send that with a short note, saying "Saw this and thought of you…". Ideally, it should be an article you have written, whether online or in print. Or, to make an impression laced with credibility, send a profile of you in a publication. But if you don’t have something impressive, any article of relevance will do.
An even more relevant way to stay in touch is to send a case study of a successful project you have just completed, or simply the highlights. Don’t get hung up on trying to find or create the perfect thing. It can be a tidbit, a resource, a tiny lesson you recently learned that they might benefit from. In fact, less is often more. Remember: it’s simply an excuse to make contact with the goal of providing something relevant.