not sure how it works in the Big 4, however in my experience there are two processes:
1. the official line: liaise with HR and get staffed on projects as they determine.
2. the "real" method: get chummy with the seniors and let them know you would like to work with them on their projects.
I think you need really to use HR and follow the official process in case internal networking doesn't work, but definitely do the internal networking too as that's the quickest route to getting staffed on the projects that you want to get staffed on.
My best advice would be to find somebody reasonably senior who is willing to take you under their wing and who will pull you onto his/her projects (or ones that he/she is working on). You can usually be quite direct about asking somebody to be your mentor - people are usually quite flattered by it and are usually happy to help.