Decorate an elementary school classroom according to the theme of the unit. For example, if your theme is spring, hang up pictures and posters with the trappings of that season (flowers, baby chicks and sunshine). This kind of visual allusion to the unit's theme works effectively with small children, who will begin thinking about the subject as soon as they walk in the door.
Read an excerpt from a text relevant to the theme of your unit. For example, if you will be studying various conceptions of heroes and heroism in literature, provide students with an essay or piece of prose or poetry that discusses what constitutes a hero. Reading about the concept provokes students to begin considering the theme.
Play a film or film clip, YouTube video or song that addresses the theme of your unit. For instance, if your class will be looking at different types of media and their effects, you might play a popular commercial to introduce your unit.
Discuss a concept integral to your thematic unit as a class. For a thematic unit on heroism, write the following statement on the board: "Heroes do mighty deeds." Discuss the truth of such a claim, encouraging students to consider the concept of an unlikely hero. Ask them if there is such thing as a "hero of small deeds." Brainstorm a list of characteristics of a hero.
Visit a place connected to the theme of the unit you are studying. Visit a local museum to kick-start a history unit, or an art gallery to instil in students an appreciation for art, before they actually try their hands at it.
Assign a writing piece or other activity for homework that prompts students to start thinking about the theme. Instruct them to interview their family or friends or search the Internet or magazines to gather popular opinion on a particular issue. Assign a personal essay in response to the theme of the unit.