I havent yet but will be doing it in the new year. My case is slightly different as Im doing it for a good friend so dont need to be matched up.
The main thing to decide is what kind of surrogate you are happy to be. You can either be a host surrogate where you use your own eggs (which is what I will be doing) or where you have an embryo implanted that has no biological connection to you.
There are organisations that can help match you with a couple as you are not allowed to advertise publically that you are a willing surrogate, there are guidelines and tests to have before being approved. If you have a google you will be able to find alot more information.