Keep a record of what you're writing!
Years ago, a creative writing teacher told me about William Faulkner's method of keeping up with his characters. Faulkner whitewashed a wall of his workroom and wrote the various characters' names and relationships on it as he worked. When he'd finished the book, he whitewashed the wall again. Given the density of his prose, I could certainly understand his using this method!
Now, since I've created a whole universe, the lesson's come home in a big way. I use the Excel expertise I built in the office to solve my problem. The character list at the front of BEFRIENDING ALIENS is a spreadsheet converted to Word. I wish I had done this for A TEST OF ALIEN ALLIANCE; the list makes it much easier for people to follow the story.
Now that I've helped my readers, I decided I'd better help myself. I have a spreadsheet with a list of all the planets I've thought up, who lives on them, and a little bit about them. In the same workbook I've set up a spreadsheet of all the space ships I've mentioned in both books, who controls them, and what they are used for. This has proved to be a wonderful resource, and I'm going to have to add a character page for my current work.
If you write a series, as I'm attempting, it is horribly easy to forget people's names and the planets they live on. Almost every series author has slipped up this way at one time or another. Terry Pratchett has the best cover-up; he says they're just alternate realities! Anyway, you can really get your fans confused.
Name mistakes like this are often subtle enough to pass unnoticed by proofreaders. I discovered one in BEFRIENDING ALIENS that really bugs me and am hoping nobody notices it. Tell me if you find it.