Be aware that a clogged clothes dryer duct will reduce the efficiency of the clothes dryer, and could start a fire.
If your duct is made of ridged metal, it is rather easy to clean because you can snake brushes, even rags right through from one side to the other. Terry commented on this article with the description of his dryer ducting, after having trouble with flex duct. He made it all with rigid metal ducting and even used aluminum tape rather than screws because lint gets stuck on the screws. Terry has in fact succeeded in making a smooth unobstructed duct. If in addition you use two 45 degree elbows rather than one 90 degree elbow, the longer slower curve will move air even better. In fact, click here for a quick look at air flow resistance in ductwork.
If your clothes dryer duct work is made of flexible ducting, plastic or metal, it is a bit more difficult because things tend to get stuck. If it is a straight run, simply take a toilet brush, bend it if necessary to the right size to barely fill the duct, attach it to the end of a broom stick and the trick is done.
If there are a lot of curves, take a bottle brush, bend it to the right shape. Then find a way to snake a light rope through the ductwork, maybe starting with a semi-flexible electrical wire or even a garden hose to get something right through the ductwork, then pull back the rope, then pull through the brush.
When nothing else works, you need to take the duct out, pull it straight and clean it out.
There is a great new air flow measurement instrument, made specifically to know when lint is clogging your clothes dryer -- LintAlert. One model even has WiFi e-mail alerts. You can find it at most renovation centres for a bit under $70. It is an insurance policy for things that we totally forget, and even if it doesn't get to starting a fire, the clogged duct will increase the cost of drying your clothes because you are putting in the same heat but having less air flow.
If your duct is too long you can add booster fans to the duct, but only ones designed to handle the lint so that they do not become an fire hazard. One specifically made for this job can be found at Continental Fan.com.
Follow this link for details on stopping cold air drafts coming in through the clothes dryer.