Distant landscapes often appear hazy, washed-out and blue due to Rayleigh scattering.
What can be done to maximise the contrast range of the exposure when photographing such scenes?
For example:
You should be able to decrease the haze a bit using a polarizer or haze filter.
Additional Information on Haze filters
A haze filter absorbs a greater about of UV light than a normal UV filter can.
For Tiffen Haze filters:
Haze 1 - Reduces excessive blue haze caused by UV light by absorbing 71% of UV. Great all-around UV control.
Haze 2A - Absorbs all UV light; reduces haze; maintains color and image clarity. Best for high altitude and marine scenes.
As has been mentioned here a few times, a UV/Haze filter or a Polarizing filter will help mitigate the effects of haze. There are quite a wide variety of UV/Haze and Polarizing filters, and there are several other topics here on Photo.SE that discuss them, their pros/cons, etc. so I won't go into that here.
To take a different tack than everyone else, another option is to try to incorporate haze and the fading it causes as distance increases into your photography. While haze is often viewed as a detractor, if you use it as a tool, it can be very beneficial, and a powerful artistic tool. It can also help you find a better angle to photograph your landscapes at, bringing greater and more visible depth into more compositionally appealing scenes.
For example, I took the shot below from about 13,500 feet up on Mount Evans, Colorado, shortly after Labor Day this year (2010). A couple days earlier, and continuing into that day, some bad fires had raged along the Colorado Front Range, putting a lot of particulate in the are that caused some terrible, thick haze that created a total whiteout during the day. I had to wait until just after the sun had set to cut through most of it, but the remainder I actually liked, as it helped add a needed element of depth and differentiation to the scene:
While haze is normally a problem, and can often be from light dispersed so much that all you can capture on film is a frame of glaring white, with a little filtration and some artistic vision, you can incorporate even haze into your photography. Some of the toughest things about landscape photography are learning to wait for the right time, and to take what you get. When you turn the tables on haze, it can become a powerful artistic element in landscape photography.
I always use a polarizer when shooting landscapes. This helps straighten the light and "clean up" the haze. Utilizing the zone system is also helpful in increasing dynamic range. I find that "haze" is highest during the middle of the day, when the sun is closest to its zenith. Shooting during magic hour will give you less Rayleigh Scatter AND will provide better lighting conditions all around.
Happy Hunting.
Apart from filters (which are the best way to minimise haze, chills' answer covers that base nicely) anything you can do to minimize noise (get as much light in as possible (e.g with a longer exposure), expose to the right) will help recover contrast lost to haze when post processing. This is because increasing contrast in post will increase noise, and the fact that noise and dynamic range are inverses of each other.
Whilst air molecules themselves produce Rayleigh scattering (making it unavoidable... on Earth!) airborne particulates can make it a lot worse. As such haze is often minimized after a storm or strong wind has cleared particles. Humidity can also be to blame as particles act as nucleation sites for water molecules to form around, so haze can be worse in humid conditions (so called wet haze). If you're watching the weather forecast look for high pressure immediately following a front. If the high sets in for too long haze builds up due to lack of wind.
Your location will also affect haze (steer clear of large cities) shooting into the sun will also make it worse so plan your shoot around the times you expect the sun to be behind you.
