The one trick I've learned regarding inside corners is to do one inside corner at a time. After that one side dries and hardens, you can mud the other inside corner. The TRICK is that it's EASIER to 'finish' one inside corner when the other inside corner ISN'T wet. Hence, it looks great!
If you, or someone you know, knows how to do WEB searches, use the following phrase in the search: how to install drywall A couple of hits included the following: