Legs keep sinking while swimming.

Push the head and your chest down in the water. Make sure you are looking down, not ahead of you. Your legs should raise. If not, the only thing I can think of is kick a little harder.

Wear a wetsuit!!!

Couldn't resist, just push down on your chest and try front quadrant swimming with one hand out in front all the time. This helps make you longer and balance better.

If you have no success, Ralphs will be a wetsuit race so your legs will float, you could develop upper body and simulate a race position by using a pull buoy in the pool.

See you at Ralphs

It is all about balance. Put more pressure down on your head/chest...also you can use a pull buoy to help with this...

ditto the other comments and also your forward arm position can affect your leg position. The TI video illustrates this pretty well. Some people have better ballance with their arm in the 4:30 or 5:00 position than in the staight 3:00 position, so as your arm sits lower in the water it helps pull your lower body higher. 

pressing your buoy and looking down instead of out in front of you really works. It took me a while to get it, but once you do you will feel the diffrence. Do you have the Total Imersion Swimming book???

I'll echo what the others have said. Keep head and chest down, but be particularly conscious of  getting the chest down in the water.  It doesn't necessarily follow the head.  I found it made a big difference.

Kick them See you in March!

I had the same problem until I started doing all of my kicking drills (except breastroke) on my back, without a board. It's hard at first, but you really feel it.

Also, doing dolphin kick on your back is supposed to be really good for your abs.

I have the same problem. I bought the Total Immersion book, and I can't get past the first drill (floating on my back) because my bottom half sinks. I'll keep trying at it until I get it right, and will gladly share what I learn.

Keep practicng, the TI video helped tremendusly and I finally did what felt like a total exaggeration, swimming downhill, as TI says, and lo & behold, up comes the legs, what a wonderful feeling it is when it finally comes together - I nearly doubled my previous distance yesterday - I am doing the Memphis in May Oly distance and I finally feel I will do the swim very comfortably, maybe not fast but also not having my HR thru the roof & emerge screaming "Oxygen, Oxygen, give me Oxygen"