one thing that has helped me more than the typical answer you're going to get (swim more) is to really focus a lot of energy on your kicks. Keeping your back half straight and up will take a lot of effort from your top half to keep yourself moving (afloat).

Here is a simple drill. Get a kick boad (like a mini body board) and hold it with your finger tips and thumbs out in front of you so your arms are as straight as they can be. Tuck your head so your chin is at your chest and just focus on an even kick. When you breath just lift your head straight out of the water. That will ultimately make your back end drop, this will get you better at getting back to a normal kicking motion when you lift your head to sight.

We can talk in private message some about more drills but i'm at work now and will likely not be able to respond to any for a while. Keep in touch with any questions.