Another thing I have learned through the editing process is that if and when you promise your reader something, which is called a plot promise, you better show them that thing you promised.
For example, throughout the first three chapters my MC is talking about taking his wife to Tahiti. The next chapter I mention that they went and had a good time. My editor didnt like this. They wanted me to write the scene. I admit I was just being lazy and didn't want to write it so I didn't. When my editor asked me to write it, I did as they asked and found that not only did the scene turn out really awesome but it actually became my plot point #1 when I shifted the focus of the book from one plotline to another.
Another example is later in the book the MC meets up with a friend and the editor didnt know who it was. What I didn't realize is the chapter where I introdouce the friends is one of the chapters that I cut before my editor ever saw the MS. I will go back in a previous chapter and (re) introdouce them so the reader knows who they are.
The point of this post is this, If you are going to talk about something especially more than once you better include it in the story because you have piqued the readers interest and they want to "see" it.
Think about it. If Harry Potter had a twin sister and she was only mentioned a few times in the first book and then disappeared. You, the reader, would want to know what happened with her in the other six books.
When you include something of importance is a good idea to write it down. Even if it's a Post-It-Note on your forehead. I write all over my plot notes, or leave myself notes somewhere on my MS or computer, at least it keeps me almost allways on the mark. Reread your finished MS with plot promises in mind, and let others who arent as familliar with the story read it. They will be able to see things you can't see by being farther away from the story than you are.