Hey Doc Elk,
I'm a noob too. Here are my thoughts:
1a) if you miss a shot, switch your defensive player to a player near half-court (preferably a guard, but sometimes you don't have time to be picky) and then try to guard the offensive player bringing up the ball. I use the button that makes the defensive player spread his arms out (I think it's called the "Box out" button). This causes the offensive player to usually slow down (thus letting your defensive teammates have time to settle in).
1b) if your shot is blocked, there's pretty much nothing you can do b/c no matter who you switch to, you won't stop the fast break.
2) for G/SFs that are killing you, are you using the "Box out" button (makes the defensive player spread his arms out)? That slows down those G/SFs half the time. I play with the Bulls, so when the G/SFs beat me the other half of the time, Curry or Chandler usually do a good job coming to help. Can't vouch for any other team's help defense.
3) I never go mano-a-mano against a C/PF. I usually control a guard on defense and double-team the C/PF. If you control a guard with a penchant for steals, he can usually strip the C/PF. Alternatively, you can control a SF to double-team the C/PF and use the "Box out" button to try to cause a steal without having to actually use the "Steal" button (since SFs typically aren't good stealers).
4) I do use the dunk button after posting up. But I think it depends on your player. I haven't used Duncan so I dunno about him, but I know that Eddy Curry can back down 75% of the centers in the league and then dunk on them. Tyson Chandler, OTOH, fails most of the time trying to dunk after posting up.
Hope that helps...