help me ditch the pull-buoy!

Bolded part usually has a lot to do with it. If you really are just letting your legs hang behind with the pull buoy, you should have the control to move your body some, but will need just a bit of practice to understand what it feels like. Press the upper part of your chest down.

And for your kick, don't kick much. Subtle, but with some frequency. Most try to do to much, though may not realize it. Feels like your leg is barely moving. Often it's better to do less and learn to balance than kick harder. Especially if not very good as it can do more to slow you down than help keep you up.

What you're describing is a very common problem, and as others have said, is usually the result of poor balance.  I like the kayak drill as a starting point for learning good balance.  Here's a video that shows you what to do:

http://www.youtube.com/watch?v=wDM6s0JpwnI

You only float because of the air in your lungs, left on their own your legs will fall down and drag. There are two things that in combination work to avoid this: A strong core and a good kick. 

A strong core is needed to maintain a good body position, keeping your legs in line with the upper body. I see swimmers who bend in their hips or lower back so while the upper body is well positioned, legs sink down. Straighten up to create an unbroken straight line from head to toe, such that along that axis your body works as a rigid rod you need to use your abdomen and back muscles. You should look forward, not down, but forward at around 45° angle, so you can see where you're going. This is nothing different from what you do on your bike. When you look down, you relax your neck and that transmits to the rest of your body so you can't hold a good position. 

Next thing is the kick: The kick starts in the hip and the leg is kept straight, many beginners bend their knees. You should practice your kick, use a kick board, it's good to do drills with fins to build strength too, but don't use fins more than 50% of the time or you'll get dependent on them. When you start your kick drills, you'll find it easier to put your upper body weight on the kick board, as you get better, just hold the board in front of you, your lungs will keep your upper body afloat. 

And ditch that pull buoy, the pull buoy is not a floating device, it's a training aid. You should not use any training accessory more than 50% of the time or you'll get dependent on it. 

Many thanks for all the responses guys! I am going to put all of these tips into action. I will let you know how I get on! 

Thanks again  

Work on just one at a time for awhile. Otherwise it gets confusing. One other I thought of later is to make sure your hands are cutting through the water in the downswing (the first part of the stroke after the extension forward). You should not be feeling a lot of pressure on your palm & forearm yet. You may not have learned this yet if you use the pull-buoy a lot, as it can cover that up. I suggest looking at the other items first, as they're likely easier to visualize from the descriptions alone.

Ok guys, first swim since all the tips..... and it was brilliant! I swam well over 1000m in my session today and that included 2 x 250m stints.... all without the pull buoy! thats the best I have ever done. 

I took on board all the tips about balance and body position etc but the key for me was watching a video on youtube of that total immersion guy doing a "superman" in the pool. 

I done a few of these today before my swim and this let me actually feel what the correct body position felt like. It's fine being told to lean forward on your chest or raise your hips etc etc but to actually feel it made a big difference! And if I was to describe what I done it pretty much is pushing my chest forward and down slightly....

I then let my body try and get back to this position every stroke and this highlighted that I had also been letting my hands enter the water in front of my head instead of straight out superman style. This in turn also seemed to let me work less with my arms and not tire as quickly. 

Once again thanks everyone for the help and I'll keep you updated of my progress. 

I really hope Superman can save the day for others too!  

I'm just now in the process of overhauling my whole swim technique and I can give you a few pointers that I discovered were big problems for me:

1. Kicking is primarily to keep your legs up and streamlined behind you, not so much for propulsion. Too much effort kicking will just tire you out quicker. What has been working very well for me is trying to get my legs in synch with my arms so that I'm kicking once for each stroke. Each time I stroke with an arm I give a little kick with a leg. This is different from my old technique of just kicking constantly regardless of what my arms were doing. I found that I was dividing my body between arms and legs, and they were operating independent of each other. What I discovered was that I needed to swim more with a mindset of left side and right side rather than arms and legs.

2. Your whole body needs to rotate with each stroke. I was not rotating much at all in the past. I was just keeping my shoulders and hips parallel with the water the whole time. Rotating seems to do a couple of things: it helps you get a longer smoother stroke, and it seems to help engage the bigger stronger back muscles rather than the smaller shoulder muscles. It also helps with tip #1, the rotation encourages a kick that is synchronized with the stroke (the kick helps drive the rotation).

3. Relax! I was already pretty relaxed in the water, but where I've found I had trouble was toward the end of a long workout (say around 1800 meters into a 2500 meter workout) when my muscles were getting fatigued my form and technique would start to get rigid and labored. My subconscious reaction to the diminishing muscle control that comes with fatigue was to tense up and be more rigid. When I find that I feel like I'm working harder than I should be and my breathing rate is climbing, I have to make a conscious effort to relax and kind of hit the reset button and go back to the basics [relax, rotate, etc.]. This makes a tremendous difference in keeping fatigue from ruining a good pace.

4. Beating the water just makes it mad. Whether we're talking about stroking or kicking, your movements need to be smooth. Slapping the water with your hands or kicking big splashes just serves to stir up waves in the water and make your swim even harder (and is annoying to those swimming around you). Big splashes and lots of wake don't propel you, it's just energy being transformed into splashing that could be used to propel you forward.

Good advice. I'm trying to train myself not to do this right now. I know it's a problem, I've just got to figure out how to get myself not to do it. Sorry I can't offer any advice on how to correct this problem, but if you have a problem with it you can at least feel better knowing you're not alone!

OK, latest update since my last post on 21/8:

I jumped in the pool today and knocked out a continuous 750m with no problems at all. Felt great and could have done a lot more( Indeed I did and followed it with a few "sprint" 200 and 100s)  

I’d like to thank everyone here who helped me. I pretty much used all the advice given and would summarise in order of importance below. Maybe this can help someone else like me:

• Balance – Head down, Chest leaning forward, “Superman” TI  youtube video helped a lot here. I was totally unaware of the importance of this before I read this forum.  
• Relaxing – Learning to open my fingers, relax my shoulders and arms and put  very little “effort” into my stroke was a very difficult thing to grasp – it didn’t seem to make sense.
• Rotating – Rotating my body/shoulders each stroke was another thing that I was not aware of its importance. I guess most of the power and forward momentum is generated here (ie not by using my shoulders/arm muscles to pull me through the water)

The two things I didn’t really work on but just seems to come naturally with the above were:

• Breathing – was much easier once all the above was in place. Still lots to work on here though, but Im not gasping for breath anymore, and I can actually swim for relatively long distances breathing every 4^th stroke now!
• Kick – I pretty much do a little kick every arm stroke and that’s it, don’t really think about it. I sometimes go a while without kicking at all.

So, in summation, this forum has pretty much helped me solve my heavy legs and reliance on the pull buoy ! My race is in two weeks and I am feeling great !

I’ll see you all in a couple of months when I start posting about how to get faster !

good advice. I've had to work on this.