I consider myself to be a somewhat succesful cap. So I offer the following advice which is worth exactly what you paid for it.

1) When my cap asks for suggestions, I respond, "A cohesive plan, please" A clear plan is the single most important thing a cap can do. This is more important than giving out the correct units. Even a flawed plan is better then, "I don't know, do whatever you guys want."

2) Get the units out fast. This allows time for adjustments, re-assignments, and clarification on what the plan is. 

3) Keep very few units for yourself. A general has to be able to look at the whole picture and issue commands.

4) Let your good players do what they do best. Keep an eye on the players you worry about. 

5) Detaching units for small details can be faster and more effective than telling players what to do. Players, when I take a unit from you, it is because I have some mundane task for it and I want you concentrating on the bigger task at hand.

6) Try not to micromanage. This is really hard for me but it is important. A micro managed unit sometimes acts like it has really bad AI. First, you send it there, then you player sends it here, then you send it there....What happens is that niether objective get's done and the unit dies halfway between where the cap wanted it and where the player wanted it. Either let the player use it or detach it.

7) Use command presence. Be the boss. This game is very militaristic in nature and demands a command presence to effectively lead. This may sound like overkill, but hopefully you read "What is Fun?" so you know how I feel about this subject :)

8) When you lose, it's the cap's fault. When you win, it's the teams victory.