Do you sometimes fill your stories with great detail about your characters? Things such as their physical descriptions, where they were born, everything they like or don't like? While this may seem to be the best way for your reader to get to know those characters, you may be giving too much information and bogging your reader down with description.
Try, instead, to create a character notebook. Put all those details that end up in your stories into the notebook. Have a chapter for each of your characters. Here is where you will put all the information about what makes your character who he or she is. By having all this information in one place, Shelia won't have blue eyes in Chapter 1 and green eyes in Chapter 15. Mark won't speak with a lisp in Chapter 5 and have a strong oratory voice in Chapter 25.
Make sure you put in physical characteristics, but also include background information such as what are his or her likes and dislikes. Where did your character grow up? What kind of parents, playmates, siblings did he or she have? All of these things will determine who your character has become.
When it comes time to write your story, however, you don't need all of these characteristics dumped on the reader. Winnow out what makes your character different and memorable. Having a stutter would change how your character interacts with others and the confidence he might have. If your character has a large visible birthmark, she probably isn't going to compete in a beauty pageant. Was there some incident in your character's past, an abusive parent, or a parent or siblings death, which changes how she views the world. These small details are the ones to include.
Remember sometimes less is more.