For me, writing the external conflict is easy. It's the big picture idea that suddenly comes to me and gets me all excited about a new WiP. But, as a reader, it's the internal conflict that keeps me glued to my favorite books and emotionally invested in the story. One of my favorite series (Terry Goodkind's The Sword of Truth) is packed full of external action, but also manages to create compelling internal conflict for almost all characters in the story (even the bad guys).
So, what is internal conflict? It's the internal battle(s) going on inside each character and the way the writer reveals the character arc/growth. The internal conflict should play off of the external conflict, but doesn't always need to correlate one-to-one. And plot points can be used to give a character what he thinks he wants, although it does nothing to move the character closer to the external goal. Sometimes, the whole point might be to allow your character to discover that he didn't actually want what he thought he wanted.
The best advice I've found on how to effectively use internal conflict comes from Fiction Factor:
Find the main character's Achilles' heel (the root source of his/her internal conflict) and stomp it (external conflict).