How to run AC cheaply ?

First you want an "inverter" style air conditioner!   They are more energy efficient.

Always use ceiling fans when you run your a/c.  It helps you feel cooler so you use  higher temps on the air con units.

In all my bedrooms I use black out curtains,  they stay closed  24/7 to keep out the heat and the light!  I keep my bedroom door shut all the time!  My bedroom stays cooler that way and I use less a/c. 

Hope those  help.