From my experience, the best way to stay in contact with PSAs and CSAs in SFI business is through the weekly mailer as recommended by SFI. This is because the mailer is the best guarantee you have that your message will be delivered as sent. You can send bulletins, tips, motivation articles and general howdy to your PSAs and CSAs at least once a week. I think once a week is about ideal as SFI recommends because i believe sincerely that they must have a lot of reasons and some proven experience to back their recommendations.
In this business, you may however strike very personal relationships with either some of your PSAs or CSAs as I have done. In that case, you can get in touch more often either by email or even telephone contacts. If however it is strictly business relationship, once or twice a week to my mind is about OK. That is my own take on this issue.