As with any role, the best way to demonstrate your worth is to develop a portfolio. Give talks at every meetup you can. Make sure they're video'd - watch them back and learn from your mistakes.
Get a name for yourself on StackOverflow (or whatever) as someone who can clearly and concisely explain a solution.
Write dozens of blog posts - and get them syndicated - showing how you can write tutorials, answer questions, and explain moderately difficult concepts to a lay audience.
Learn how to organise events. Start your own meetups / hackdays / whatever. Organising venues, catering, invites, sponsors, speakers, schwag etc is absolutely invaluable experience. Join the organising committee of your local events.
Finally, find a company (preferably with some funding) who you can see needs to get their message out there. Write up your plan on how you would help them succeed. Which meetups are they best to sponsor? Which merchandise has the best response among developers? What parts of their proposition need the most explaining. Pitch your experience.
A word of warning. I quit the evangelist game because I couldn't cope with frequent international travel. It's fun at first - but can play havoc with your personal relationships.