Here's what I did and I'd like to think it worked well. Main thing is to stay consistent and to not get injured, and you will surprise yourself with the outcomes. I was at 1:00 my sophomore year, 56 my junior year, then 51 my senior year. I've ran a 49.xx since then. Here is what I did during the summer months:
-Lift 3 times a week over the summer. Monday-Legs, Wednesday-Upperbody, Friday-Legs. You want to know the back squat, lunges, Bulgarian split squat, bench press, and hamstring curls. These were the main exercises I did. Start basic with 3x10 for each with appropriate weight. Don't go too heavy right away, work up to it...otherwise, you'll get injured. Form is KEY before getting heavier in weight, especially for the back squat. Have someone help you who knows what they're doing or start with just goblet squats. Do some core work as well once or twice a week. Sit-ups and push-ups work. Stretch before you do these workouts.
-Do a speed workout once a week. Suggestions: Flying 30s, 60s from a standing start, etc. You want full recovery and a max amount of repetitions at 6. If you start to get slower, either increase recovery time or stop the workout right away. Focus on form with your arms and staying relaxed. Also, don't do this the same day as a leg day. This could go on like the day you do upper body or the day before legs. Try to do some hill sprints too if you can. I didn't do them a lot but I feel like they help.
-Every 7 days do some 150s/200s. Start by doing sprint 50, float 50, sprint 50. Research what "floating" is if you're unsure. I'm sure you know, but if you do not it is an essential aspect to sprinting and the 400m for sure. The second 100m of the 400 is considered the float phase. Focus on staying fast, but not trying to get faster, if that makes sense. You're trying to maintain the speed you built up basically.
-Longish runs. I know some people who would disagree with me, but the 400m is at least 40% aerobic. I would run 6-8 miles every week on sundays as my long run, but I also did cross country. You could probably get away with 4-6 though. These help build endurance, and you will see gains in your 400. Endurance helps sprint times for those who are not naturally gifted with speed (me and you) I wasn't even good at XC, my best 5k was mid 18s but I had the best speed on the team.
-Lastly, plyometrics/drills. Do some vertical jumps, one leg skips, broad jumps, and bounding twice a week. Helps reinforce the fast twitch muscle fibers and your explosiveness. Do high knees, butt kicks, a skips, etc, before your workouts.
This is just the basics. Getting good sleep, nutrition and other things all play a part. Stay away from stuff that gives you high amount of lactic acid. The intervals that make you throw up, the 300s, etc. save that for preseason work. I would do things like 150, 300 time trials every 4-6 weeks. Just to give you an idea of where you are before preseason work.
Still, have fun with it. It'll be the summer so Have some friends do the workouts with you, cross train in the pool, etc.
Good luck.