The following video defines hypermiling in this way: “To take extraordinary measures toward achieving maximum fuel efficiency in an automobile.” Using a Fusion hybrid vehicle, they reach more than 1,000 miles on a single tank of gas (about 84 miles per gallon):

This guy is able to get 50 MPG from a normal Honda Accord:

So how do you do it? The following video discusses several techniques:
1) Drive calmly, less aggressively. Gentle acceleration and braking
2) Don’t idle. Shut off the engine
3) Anticipate traffic conditions
4) Drive the speed limit (or less if you are not impeding anyone)
5) Coast when you can
6) Park on hills for a rolling start
7) Use maximum air pressure in tires
8 ) Clean the air filter
9) Use hood insulation
10) No A/C, windows up

More tips:

See also: Hypermiling (Saving Lots of Fuel) in a Hybrid