It is good to start with a good file naming convention for your digital images and stick with it, one that will be useful as you go on. A common one that I have used is YYYYMMDD_XXX. XXX is the file sequence for that day. For example, my first shot for today would be 20130130_001. Most image management software will ease this process, some will automate it. By the way, I have yet to shoot over 999 images in a day.