A call back in comedy is a technique that takes something that was said before and ties it back into the current topic. The reminder results in a clever connection and can create some great humor.
A convention theme can have the same effect by tying together common themes at breaks, the keynote speaker, break out speakers and the after dinner speaker.
Simply take variations of your convention theme and have conference speakers call it back throughout the day.
When you reinforce your theme it creates a congruence and point to the event. The theme should not be an afterthought but deliberate connections weaved throughout the convention.
Set the stage at the opening keynote session by explaining the theme, why you chose it and what it means to delegates. Ask the keynote speaker to relate her message to the theme. You could have a panel discussion or facilitated session that discusses and digests different elements of the theme.
Publish the theme through twitter early on to generate input on conference speakers and sessions.
Find relevant sponsor organizations to back the theme (ie-a green theme would be ideal for a green organization to sponsor)
In the conference brochure have the convention chair write a welcome letter to delegates highlighting the theme
In convention delegate bags, put flyers and materials relevant to the theme
Give gifts to the conference speakers, keynote speaker and after dinner speaker relating to the theme.
Create dinner table centerpieces related to the theme
Have interactive games or quizzes that generate conversation around the theme. (a treasure hunt, crossword puzzle, etc tying in the theme
When thanking a keynote speaker or after dinner speaker relate learning points to the theme
Have follow up through your association newsletter, blog and twitter discussing the theme
Continually Banging people over the head with a theme has the added benefit of it having a lasting impact.