What should you write about?
Every press release should center around an event. Such as a launch, a new feature that has just been released, the 1.000.000th visitor, results of a survey you published today, etc.
How often should you issue a press release?
You can issue as many releases as you want, but you probably don't want to send a release to the same contact more than once every 2-4 weeks. You should also consider the newsworthiness of your releases.