We love to write. The creative juices start flowing and a world comes a live. Characters become more then words, you can see yourself in the places they are, the situations they find themselves in. You want to follow the story and see what happens. It's all a wonderful feeling. The problem with this is that once you head down a path, you are kinda stuck if you publish one book at a time. So here are a few ideas I might suggest.
1) Before you publish that first book you know is going to be a series, at least have an outline of the whole series. Better yet, finish them all then publish one at a time. Either way, you have exactly where you want to go and not where the book leads you.
2) By having at least an outline you can weave stuff in. I dislike when books or tv shows try to say person A really did love person B without any back ground to back that up. Or where did that bad guy come from and do they really have a reason to be the bad guy. As I wrote my following three books, I had to go back and weave in some details because I came up with some good ideas as I went. That isn't bad, just you have to make sure it works.
3) Have someone else check to make sure it all makes sense. This really goes with some one editing your book, but if you just have a friend read through it you can ask them to be brutally honest on if it flows. It might hurt to hear some of their critiques, but it will help you be a better writer.
Happy Writing.