How do airlines create their flight numbers?

I notice that often the same routes have the same flight number, but sometimes they change, are they arbitrary?How do airlines create their flight numbers?
I think they are pretty random. I have noticed as well that the same flights keep the same numbers for a while, but I don't think that airlines seem to have any method for picking a number. One thing I have noticed however is that short flights, like those run on commuter jets, usually seem to have 4 digit flight numbers, while longer flights run on jets seem to usually have 2 or 3 digit flight numbers.