How do you deal with rude customers?

I fight what I really want to say and do because I have to...businesses now make it part of your job description to lay down like a floor mat and let them walk all over you. Customers can come into the place of business you work and treat you any way they want, and you can't say or do a thing about it because "the customer is always right". Anything for that almighty dollar including welcoming the mistreatment of employees in order to keep them coming back to spend more. It's BS and highly stressful.