I'd say the number one thing I see people do is pulling and kicking at the same time, or nearly the same time. Then when they have lag time between strokes they start to sink so they have to pull and kick again quickly. this can be very tiring and not efficient at all. I always say "Kick, Glide, Pull, Breathe". During the glide phase you need to be in a streamline position (head down). You are literally being stretched from fingers to toes. the glide is as important as any other part of the stoke, even though you're not moving for a second or two.
The other thing people do is pull down to their legs with each stroke. Imagine a big mixing bowl right at your chest and don't go wider than that (just outside your shoulders) keep your elbows UP as your hands pull around towards your body. They should be just at the bottom of your rib cage, no lower.
One more thing. As you come up for the breath your legs should be starting to set up for that kick so that as soon as you take the breath you're ready to kick again with no dead time.
maybe that was confusing...I'd google some small video clips if I were you