Here's what I'd do:
-Prepare a summary with all the points I want to bring up
-Include my sources, links to sites, etc to share
-Bring sample products
-Contact your local congressperson for help preparing (call, or it'll take ages for a response)
-Practice, practice, practice my responses so that I'm calm and cool when the time comes
-Dress professionally (a smart business suit or sweater and skirt combo)
But you sound like you already got some great advice!