Once is rarely enough; repeat, repeat, repeat is typically the rule. The gravity of the message dictates the amount of repetition. It is important to repeat the message frequently and in new and different ways. Varying the media used can assist in this effort. Use video for one announcement. Choose a meeting for a reiteration of the message. Post banners for thematic tie-ins. And use email to reinforce key points. In this way, the recipient receives the message in a variety of different ways over a period of time.
A word about budget: Video and print can be expensive, but in-house production facilities can reduce the cost. Furthermore, you can use other forms of media, like articles and webcasts, to carry the bulk of the message and be selective with more expensive media.