You shouldn't try to swimm downhill, but should feel as if you're swimming downhill. The distinction is important. A good way to acheive this sensation is to "press the bouy." This means to press your chest down into the water. Similar to the position you'd acheive if you float face down and someone pushed down on your shoulder blades. Another important aspect of swimming is to roll. When rolling properly, at first you'll feel as if you're going to flip over onto your back... and it will happen a few times, but don't sweat it. Try the techniques found in Laughlin's Total Immersion.

I've done it a couple times just as a mindset thing.  It puts a spring in my stroke up until I turn, then I can't shake the thought that I am swimming uphill.  So I quickly put the whole concept out of my head.

I highly recommend the Freestyle Made Easy DVD and any of the TI books that demonstrates and discusses this in more detail.  The main ideas are balance, streamlining, and efficient, effortless strokes.  There's a lot to discuss but balance is the main issue when discussing this topic.  For many people, their legs sink.  By "swimming downhill", you're achieving a balance between pushing your upper body downward which counters by keeping your legs up and not sinking.  Achieving this balance is a step towards streamlining yourself.  Check out TI at www.totalimmersion.net.