How do schools choose textbooks

[psychobyte]

Hi,

I'm thinking of getting into the online textbooks market. I was wondering if anyone has any knowledge about how professors/teachers/districts choose what textbooks they will use?
How much do they(high school and below) usually spend per purchase or per year?

Also, what strategies or marketing do textbook authors use to get schools to use their books? Do they just "write and pray" that their book will get noticed but instructors?

Thanks.