This simply isn't true unfortunately. The ISS is constantly losing velocity due to atmospheric drag, while the atmosphere is extremely thin in comparison to sea level at ~370km LEO it is still enough to reduce the apsis by up to 100m per day.

There are two types of reboost - single burn and two-burn. A single burn reboost involves one firing of the thrusters. The impact of the firing is an increase in altitude on the opposite side of the planet. This type of reboost is done for small reboosts because it does change the eccentricity of the orbit.

http://i.imgur.com/ApeQoDv.png

A two burn reboost essentially starts like a single burn reboost, but at the 180 degree point it fires the thrusters again to cancel out the original delta-v. This results in the ISS being in a new circular orbit at the altitude of the second burn.

http://i.imgur.com/9csaPK7.png

The design envelope of the ISS is to keep it between 280 km and 460 km. But we don't usually reboost up to 460 km and then drift down to 280 km. The reason for that is that we don't want to make the visiting vehicles work so hard and burn so much fuel to get up to 460 km.

So, that means we do smaller, more frequent reboosts. They occur about once a month and involve a delta-v of about 2m/s and involves firing thrusters for about 900 seconds, although that is variable depending on which module does the burn.

Most often, reboosts are done by an attached Progress module. We usually use the smaller thrusters on the vehicle, because we do not want the acceleration on the vehicle to greatly affect ongoing payload science. Typically we use four thrusters that each have a force of 13.3 kg-f (29.3 lbf).

The propellants used are UDMH and NTO (Unsymmetrical dimethyl hydrazine and nitrogen tetroxide).

http://i.imgur.com/7MS3Im2.png

^{credits to Madan Moham}