Having done the job myself, after trying stuff on I will always check with the assistant if she would like me to put the clothes back myself or if she wants has to do it.
If im in a shop I find myself tidying the rail [ie sorting/hanging items in size order!] because once you have done the job the 'rules' never leave you!
I found that a cheery attitude and a smiley face despite the utterly shit pay went a long way and on the whole customers would reciprocate.
Yes you did get the odd rude customer, but I still kept the smile on my face and 'professional' [Im fully aware that sales assistant isnt a profession] stance as I was being paid to do a job and represent the company, if I had been rude back then by sods law that would be the customer that would have taken the time to give head office a call!
I do accept though that sometimes company policy dictates how you are with customers, the company I worked for had a policy of doing whatever the customer required [ie they can take as many items as they liked into the chaging room and if they needed another size I had to go and get it for them!] I know that some shops will only allow 4 items max in the changing room.
The company I worked for also required insisted that we approach customers 'Hello, do you need any help' that sort of thing, which I knew wasnt always appreciated, but we were just doing our job!
I on the whole enjoyed the job and several times had good feedback from head office because a customer had phoned up to say they appreciated the help I gave them.
Seriously, a smile and friendly attitude really does go a long way.