If you are interested in working for a specific practice or function do the following:
- Get yourself on the relevant newsletters and staffing lists
- Write to the respective staffing coordinators for those areas, highlighting your interest
- Network with EMs and partners in that area
- Attend relevant events if possible
- Actively ask to work on research opportunities and proposals in that area
You want to get your name out there early and while it might not work out instantly, within six months of actively pursuing these steps, you should be staffed on your first project in a certain area.
From then on, keep networking and if you perform well, word of mouth will do the rest. Most staffing is done through networking and not official staffing channels.