I seem to fizzle with 100 meters to go. Break the event up into 100-meter segments. For the first 100, get out of the blocks and into your rhythm. Lift your knees and let your momentum carry you down the backstretch. The third 100 is the critical one.
Runners tend to slow down because they're coming into a turn.
This is also a point where you should take a deep breath to relax and refocus so you won't tighten up. If you work the third 100, you'll have a slingshot effect coming off the turn to set up your final 100 on the home straight. Try to maintain your speed, as opposed to shifting into a different gear. Even though the 400 is considered a sprint, you can't just go full tilt until you run out of gas. It requires thought and planning.