Crashes are bad. Avoid crashes. However, don't just sit there. Work hard to keep the airplanes moving and the passengers happy.  

A good controller can keep everything moving seamlessly in a safe, crash-free manner. Arriving airplanes land themselves. That's the easy part. Occasionally you will notice a 'tie' between two arriving aircraft on different runways. A tie is when two planes try to be in the same place at the same time. As a highly paid air traffic controller, it's your job to make sure that never, ever happens. Use speed control (i.e. FAST, SLOW) to manage such ties. Failing that, use GO AROUND to cancel an aircraft's landing clearance, or give a 360.

Departing planes are different. It is up to you to decide when they should be given a takeoff clearance. Always ask yourself, "What are the consequences?". Look closely to see if another aircraft is about to land, or has just landed and is still exiting the runway. Don't forget to look at the other runways, too. The biggest failure point in Airport Madness is the intersection of two runways.  Taxiing planes to and from parking is easy. They won't crash into each other on the taxiways, but they will crash if they taxi onto a runway. You can mindlessly click a parked aircraft to make it taxi to the runway, but if the aircraft needs to cross a runway, you must consider whether or not the runway is being used. Arriving planes can be manipulated in a few different ways. You will notice that not all runways can be used for landing. That's just how it is. In real life, controllers establish a 'flow' where certain runways are only used for arrivals, and certain runways for departures.  

Free tip of the day: Don't be afraid to let planes wait! Crashes aren't cool. Let the planes line up on the taxiway if necessary. Tell an arrival to GO AROUND if they are about to have a runway mishap with another aircraft.  The CONTINUOUS PLAY button can be selected if you wish to play one particular level indefinitely, with no crash penalties.