Funny you should mention it....
Air force and navy require certains tests or on the job training plus evaluations to get promoted. Army and marines both promote more on a total soldier concept and award points based on various things. Army promotions to E5 and E6 are similar to Marine promotions to E4 and E5.
In the army promotions are automatic through E4...just don't do stupid stuff. For E5 and E6 you appear before a promotion board which is mostly pomp and circumstance as your board members are already going to be leaning one way or another. Just don't screw up. You've been vetted...don't make your leadership look stupid by coming in there all jacked up. Boards are mostly about how you handle stress.
Once you've gotten through the board and have your promotable status its all about points. Each month the army decides how many soldiers from each MOS they want to promote and set points accordingly on a 798 pt scale. Combat MOS's are typically lower. ..350 would be high for something like 11B. You accumulate points based on things such as APFT, weapon qualification score, # months deployed, military training, military education, civilian education, awards, badges and tabs, correspondence courses. Again, this is for E5 and E6. Above that is a lot about your evaluations, board file, and additional duties.
If you want to "play soldier" google: promotion points calculator.
Alan