Absolutely the main key is to just start DOING IT. Set up your own website and start reviewing things you use a lot and have expertise in. Put a copy of every review on Amazon. Once you get a library built, the traffic will come to you exponentially and publishers will start to find you. Then you can go promote yourself at professional websites if you want - or sometimes they will come and recruit you.
By proofreading do you mean where someone gives you a book and you give them the errors you find? LOTS of writers on the web need this type of help. Again, set up a website that explains your rates, your offers and so on. Then start hanging out on writer's forums. When someone talks about needing help with their book, offer to be that help. I'd even offer to do a few for free to build up your references page, your testimonials page and so on.
Make sure you have a Twitter, LinkedIn, etc. page set up so you are getting attention in all the social media!