Everyone seems to agree that 'communication every day' is important.. however, I would add to and modify that a little. Communication every day is not always possible and you should be aware of that. However, when you know that communication is not going to be possible (i.e. I am travelling abroad for a week, mobile costs are expensive and I am unlikely to be near a computer) you should communicate the reason for and the extent of the lack of communication. The worst thing in the world for an online sub is to wake up in the morning, expecting to find an e-mail that is always there every morning, only to find nothing. They start to worry... I know this because I break this rule all the time and, when I finally do get to log on, am met with many mails filled with concern.
So, known breaks in communication should be reported in advance and unexpected ones resolved as quickly as possible. This is another reason why many forms of communication is a good idea - when one fails, there is always a spare.