Lots of good tips here - disorganization and doing more work than the job calls for are major time contributors in my experience. This might sound weird, but don't approach a shop job like it's your own bike - that will usually lead to going the "extra mile" with a task that results in extra work being done. Example - a bike comes in for a new chain, you notice the cassette and chainrings are filthy so you go above and beyond and give them a cleaning, that adds a lot of time to the chain task.