What should I keep in mind when extending downspouts?
45 degree bends make the pipe easier to snake out if it becomes clogged. This can definitely save you a headache in the future. 

Some suggest using a catchbasin with a grate below each downspout rather than connecting the downspouts directly to the underground pipe. This gives one last chance to keep leaves and dirt out of the underground piping. 

Slope is definitely important for proper drainage. 

As far as distance from the foundation, if you choose to  use non-perforated pipe, I suppose you could run them closer to your foundation, but I'd avoid it if possible.