Having spent many years in support, the most valuable lessons I have learned is to pitch your conversation to the customer's level of experience/knowledge, to ask the right questions and never to assume that the first line checks have been completed.
More than 20 years ago I worked at a large NGO that had outsourced its IT to an external supplier. Computers were so locked down you couldn't even install a new printer without assistance from support.
They put trainees on our helpdesk and moved them to other accounts as soon as they became competent, replacing them with other newbies. The only thing you could reliably get out of them was the weather forecast for the next day or so - the helpdesk was located to the west of our office.
I eventually set my laptop up to dual boot into a copy of Windows I could actually control myself.