How do I get the assignment I really want?

Buy the branch manager a coach bag.  Works like a charm.

As has been already posted multiple times; it's about the timing. If you can substantiate your assignment's relativity to your personal career growth; you can make it happen. You need to convince your career counselor of this fact with hard evidence (NCOER's, counselings, letters of continuity, etc..). An endorsement from the 1SG / CSM is a plus. As far as the timing; before you come down on assignment you need to be in constant contact with your branch manager to establish a repertoire with him/her. Familiar "faces" are more easily recognisable, and sense you are currently not assignment / pending orders; you are fore "fluid" in this regard to placement. I would recommend contacting them 6-8 months minimum out from your projected PCS.

SSG Watson,I am a 68 CMF Soldier that had a extremely hard time getting special assignments. It went so far as the Branch NCOIC personally screening my records and telling me I was competitive for a special assignment. The timing never worked out. Fast forwad years later and many phone calls, I'm serving as an Instructor at Fort Sam. You really have to try your best to force your will. I got my career counselor  and CSM involved. Anything helps. Now might be a little hard with the cut on PCS budget. Look into Instructing, AITPSG, WTB Squad Leader/PSG and BAMC (If you are 68CMF). Keep the faith brother.