As someone who was a TPS, the best thing you can do is befriend the current TPS/ETL-AP. When there is an opening, express interest in it. If you're studying some type of law enforcement classes in college that might help too. Ultimately the ETL-AP decides if they want an internal/external candidate. Keep in mind, its much easier to go with an external one since then you don't have to worry about the potential TGT TM being "too friendly" with other TM's/ hanging out with too many TM's outside of work. AP doesn't like that since that will question if YOU can be trusted with all the sensitive information.
Another route, which I would HIGHLY recommend is going to LPJobs.com
You'll have much better opportunities finding AP type of work there and you can skip the politics game.
Best of luck and keep us posted!