Sometimes the make or break point of a youth rally can be one of the very first things set and that is the Theme. It sets the tone and creates excitement as well as gives the youth a phrase to sum up the day/event to help them remember. Over some time I have got a few ideas that could be built up and made your own and from the base of a strong rally so I thought I would start by sharing one of my favorite.
If you have a strongly male youth group or are looking to attract more guys to your event one theme that might play to them is a play on the video game series Call of Duty, a war based which may cause some problem for a few parents but you can down play the whole gun part if that problem arises. The title of your event would be Called for Duty in that Christian have been called to the work of Christian, A strong theme but leaving it open for speakers to be able to play to their strengths.
Sub-topics could as so be set-up off the game by using Game-types as titles of lessons:
Domination - Dominating fears that hold us back in spreading Christ.
Search and Destroy - Searching out Sin in our lives and Destroy sinful opportunities.
Headquarters - Setting up a home base of safety
...and so on as needed for your event.
T-shirts would be an easy fix with options jumping out from the beginning, in that you could change the cover of any of the video games boxes such as holding a bible instead of a Gun. An other option would be to do anything with someone in uniform.
Hope this helps, feel free to take and need and let me know how it goes.