You don't say what training you are currently doing, or where you are losing time?
Is it towards the end of the 400m?
Do you happen to know your 200m split?
Is this a race or during training?
If you can supply a little more information, I will try to make the answer specific to you, but with the information that I have, here is my answer.
To run a good 400m you need speed/endurance. This means that you need to run a good speed for the whole distance, but it is too far to sprint, therefore you need some endurance as well.
What I would recommend is to train once a week, under distance. Say 200m, but do lots of them, making sure your pace is faster than that of half of the 400m. So for you I would suggest around 40 seconds or there abouts. Do 5-6 200 metres with no more than a minutes rest(maximum) in between.
Then in the same week I would train over distance: 3 times 600 metres or 800metres
The first session will make you faster, the 2nd session will help you maintain pace.
A very common problem for 400m is to fade badly in the last 100m, this is caused either by lack of endurance training or starting too fast.
If you running the 400m on you own in training,you may find if you bring someone to training with you that you are able to beat your time now. There's nothing like a bit of competition to get the best out of yourself.
Regarding technique, in the last 100m or so, when you are getting tired, concentrate on keeping your running form, make sure you are pumping your arms. You could definitely gain a few seconds if you do that.
I hope that helps, and good luck with your goal