Be sure to do the following:

Keep a log - write down and date everything you do even if it is thinking 
about an idea that you later discard. Those that did well at  the regional convention
 had excellent logs and drawings.

Read the materials in you invention packet carefully and keep materials 
that you use  to list as references.You must do reserch on the Internet, 
in catalogs, in stores, etc. to see if your idea is already on the market. If it 
is, think of an IMPROVEMENT to the existing product.

Remember you will need some way of displaying and advertising your 
product. They are again awarding special prizes for the SMALLEST and 
most UNIQUE displays. If you help getting started, please use the links 

Fantastic Contraption