Ever wonder how the International Space Station stays in orbit so long?
Answer: the ISS gives itself a reboost every now and then.
ISS reboosts orbit ahead of STS-117 - Russian spacewalk preps
Visit the ISS Height Profile to see how the altitude of the ship varies over time.
The space shuttle also gives the ISS a boost when it comes up for a visit, but that doesn't happen as often as Progress vehicle visits.