The cool surfaces of an air conditioner’s refrigerant coils attract moisture. This condensation will drip into an overflow pan that is connected to a drain line.

If the moisture that air conditioners produce is not drained away, it can cause leaks and water damage within the home. The potential for water damage starts with the condensate drain line. This is how the water that condenses in an air conditioner is taken away from the unit. If the drain line gets clogged, the overflow pan may not be able to contain all of the condensation and there will be leakage.

Ways to Prevent Condensate Drain Clogs 

• Regular Flushing
Routine maintenance is an effective way to keep the line clean. The homeowner should regularly flush their lines with warm water. Some advise using bleach instead of water; however, bleach is not good idea since most drain lines are made of PVC or ABS plastic. Bleach can break down both of those materials as well as the cement used to join fittings together. Vinegar is another option as it is an effective cleaner that will not damage pipes.

• Proper Drain Line Installation
In some cases, blockages may arise because the drain lines were not installed properly. For example, lines that are improperly sloped tend to be prone to clogs. It may be necessary to get an HVAC professional to re-route the line.

• Condensate Trap Installation
A condensate trap with cleanout ports should be installed. Clogs will occur at the trap and will thus be easy to remove with a brush via the ports.

• Air Filters
A quality air filter can ensure that dust does not build up on the evaporator coils. This dust is one of the main components of clogs. It is important that the air filters be changed every few weeks as dirty air filters will not be effective.

How to Remove Clogs
The simplest way to do this is with a wet/dry vacuum. The vacuum hose should be connected to the end of the drain line. It is important that there be a tight seal to maximize the suction power. The vacuum should be turned on for approximately three minutes.

It is important to note that air conditioners can be installed in multiple configurations, some of which may make it extremely difficult or impossible to get to the drain line. In these instances, it may be best to leave maintenance and repair to a professional.

Sources
http://inspectapedia.com/aircond/Condensate_Drain_Cleanout.htm
http://www.doityourself.com/stry/prevent-clogged-air-conditioner-drain#b