I was driving downhill over the weekend at 40 in 5th gear and reset the mpg meter. For the next quarter of a mile, the it showed 99.9mpg. OK, so the sample size was a little small, but still it was nice to see!
Most modern cars should automatically cut the fuel to the engine when you coast in gear without touching the accelerator (unless your rpm drops below a certain point). Basically, if you want to keep your mpg up make sure you keep in gear any time you're going downhill or slowing down - once you take it out of gear the fuel has to cut back in to keep the engine turning.
There can be something of a tradeoff when going downhill though - if you keep in gear your fuel consumption is zero, but engine braking may slow you down to the point where you need to blip the accelerator again, whereas if you come out of gear you have to consume some fuel to keep the engine ticking over, but should coast much further (& will usually gain speed). I'm sure some of the dedicated hypermilers out there can give some much better details of the pros & cons of each approach.
The really dedicated ones turn off their engines altogether downhill, which is fine so long as you don't mind losing the power assist on your steering & brakes :w00t: