Best Practices For Large Libraries

[kurek]

What are the best practices for maximizing responsiveness and performance on a large library?      

Large means different things to different people but I assume the same techniques would apply regardless.   

Is it valuable to keep album art size small?   
Is it valuable to eliminate unrelated files from directories (like alternate album art,  text track lists, etc) ?
Do some directory structures work better than others?

I'm asking to optimize my own system but general answers are good here for others who might find this thread. 

Thanks!