Believe it or not, the inventors and developers of table-top role-playing, the grandfathers of us all, as you might say, have written several books addressing this very topic. Many of them are entitled "Dungeon Master's Guide," or something similar. You might be surprised how often the advice "Know your players" appears.
If it's true that "these are my friends and I want to continue playing with them," try talking to them like you were their friend.
