I been in the Military for 8 years and I just wanted to know How can I have more power in my career...?

I want to know how will I be able to somehow before I get to my next duty station, find a unit that needs my MOS, and have them accept me in there unit... I MEAN REALLY WHO CONTROLS WHAT UNIT YOU GO TO??? Who should I e-mail or call?? I'm really not trying to go to anymore combat units...I been in Combat Arms Unit my whole career...I been to Iraq 3 times, and now I'm in Kuwait...Just would like some helpful advice on how I can take some control on my military career.