freestyle - kick, body position

Hi, I have major issues with the freestyle swim. My instructor tells me that my body is not streamlined when I do the freestyle. It is apparently bent at my hips and waist. My kick isn't great either -- I'm fine when I start off, but after a while I can feel my kick getting weaker. Infact, while doing kick exercises with a kickboard, I seem to go no where after a while. Things are slightly better on my backstroke, but my legs sink after a while and my body isn't streamlined any more. I should probably also mention that my breaststroke is pretty good. Any tips you guys may have regarding body position and kicking will be greatly appreciated.