There are several theories about when you turn landing lights on/off.
One says that to leave them off until cleared takeoff/land shows the tower you are aware you have not received clearance.
The other is that they should be on for visibility at all times anywhere near the runway.
Personally, I favour a hybrid. I would prefer landing lights to come on after clearance to take off, then to stay on until >10,000, and come on again passing 10,000 in the descent. To leave them on when stationary on the runway (i.e. with no cooling airflow over them) tends to overheat them and result in much lower bulb life, and I believe that white strobes are sufficient for visibility on the runway. Below 10,000 it aids visibility with slower VFR traffic wandering around.
There is unconfirmed evidence (one case, I believe) of birds being attracted to landing lights, leading some people to suspect that leaving them on in the air may result long-term in a higher bird-strike rate. This is the reason for the development of the "flashing" lights.
None of the above represents my company's views - my personal opinions only!
PS - in cloud at night, of course, you would normally switch them off until VMC and about to land (e.g. Cat II/III approaches).
[This message has been edited by HugMonster (edited 24 March 2001).]