Summarize this article:

If you notice any spots around the back of your braid that look loose or in danger of falling out of the braid, then you might want to reinforce them with a couple of bobby pins.  To use a bobby pin, open up the bobby pin and then slide it over the loose piece and connect it to some of the hair on your scalp. You will probably need a couple of bobby pins for each loose strand. It is okay if the braid looks messy with a few loose strands near the front, but you don’t want it coming undone completely. If your braid looks a little thin, then you can gently tug on the sides of your braid to widen it. Start near the top the braid and grab a couple of pieces on opposite side of the braid and pull them apart slightly. Do not pull too hard or you may undo part of the braid. Just pull enough to make the braid look a little wider and looser. If you want your Katniss braid to last all day, then finish it with an all over mist of strong hairspray. Spray the braid, back of your head, sides, and loose pieces.  Make sure that you close your eyes and hold the can about 12 to 18 inches from your head.

Summary:
Add bobby pins to secure any loose back pieces. Tug the sides of the braid to widen it. Spray on some hair spray.