Great PR doesn't happen overnight. It is concerted effort over time that builds relationships, develops awareness and creates trust. Opportunities come up - and you need to be available to jump on them.
As such, unless you are running a one-off project, Waterfall will almost always recommend working together on a retained basis, whether that's over a short or long period of time.
For this, you will get services such as a day-to-day press office, basic media monitoring, highlighting of relevant articles in your trade press, writing of press releases, opinion pieces, regular meetings, strategy, planning, reporting and more - depending on your needs.