A fine question, agent! There are several options for agents who are under deep cover. Ephemeral messages, such as those left with rocks or on beaches, are often less likely to attract the attention or ire of enemy agents and concerned civilians. Small wearable objects are also a good idea; these may include pins or bracelets. Several agents have also (politely!) infiltrated message boards and comment threads to spread the word.
The sky is the limit, agent! But remember: sometimes maintaining cover is the most important thing.