i don't know what you're background in swimming is, so if you already know any of this then I mean no disrespect. It really helps if you keep your head down in the water, if you pull your head up then your butt sinks and you're swimming vertically and that really tires you out quickly. That's one thing that I see people doing wrong all the time. Also there are a couple videos on youtube on how to swim that are really good and show great technique, I highly recommend them. Here is one on freestyle/backstroke/kicking, this guy is us marine and has really good technique and a great teaching style, check out all of them if you have a chance.
http://youtube.com/watch?v=IYuSMumlUk4&feature=related