They encourage you to join a vets march if one's near you, but you can also show your support on Facebook by using their app, which joins them in a weeklong virtual march. It sets your Facebook status to genuinely support the troops each day, like:
I don't know, but I've been told, your Facebook status is worth more than gold. March online with IAVA to honor the service of new vets and show them you've got their back.
Beyond that, IAVA provides real help for the troops every day of the year, which we all gotta do more of.
Disclaimer: I've joined their board, in part since they get stuff done.