I encourage my students to use character sketches. It’s a simple worksheet with questions about your character’s details - age, height, best friend, favorite food, first job, nervous habit, etc. They can be a great resource for deciding how your character would react, what they would be wearing, and keeping track of how old they were at certain flashbacks.
There are plenty of character sketch worksheets available with a google search. Pick your favorite and use it. Here is a link to one example, but I like to tweak and add my own questions. http://www.writersdigest.com/wp-content/uploads/2011/06/30Days-Character-Sketch.pdf