1) The pieces of text you write are the player's reward for thinking of the command that calls them up. So make them rewarding. Every diegetic piece of text should have something to recommend it: a joke, a nice bit of imagery, a character insight, something.
2) The player should always have a pretty compelling reason to type something other than QUIT. The fact that something came out of your head may by itself make it fascinating to you, but you'll have to put in some other source of interest and motivation before players will find it so.
3) Get into the habit of finishing things. One of the reasons I've released so many short pieces while working on the long ones is to keep from getting discouraged at working and working and not yet having anything to show for it. One of the best pieces of advice I ever received when I was just starting to write regular fiction was to stop writing the first chapters of epic novels and to instead write a three-page story from start to finish. And then to write a ten-page story, and then a 25-page story, and then an 80-page story, and only then to think about writing a novel. Something similar might well be true for IF. Because of course, you can't write a good game if you don't complete it.