Be unpredictable, but not overly so that you switched into another genre or realm.
Inject humor.
Keep your readers thinking.

If it's a chaptered fic, do not take too long between updates! Some readers do not like waiting around. Another thing is to have at least a clear plot. You don't have to plan it all out, but a reader knows when the author is just adding filler because they have no idea what to do next. Have likable characters, or at least ones the reader can relate to. Have contrasting personalities and conflicts.

Do a cliffhanger

Well.. Give cliffhanger at the end of chapter.. But not too much.