I think drivability work is where the money is if there's such a place. Diagnostics is what we all have to do to determine the root cause for the concern but, unfortunately few customers want to pay for it. However, as vehicle systems become more complicated, charging for diagnostic time will be more routine. right now I feel diagnostic time gets written off to get the repair job price lower.
The way to make money is to not turn away anything.
Even if it's painful like a heater core on an F150, take the pain and do the job. Don't pick and choose. You best tool as others have pointed out is your brain. All the tools from the snap on catalog, a laptop, and electronic service information won't help you if you can't understand it and how to use it.
lastly, when at work be at work. Don't surf **** on the internet on your laptop, live without facebook, texting, or talking on the phone. Be 110% focused on work and repairing cars right the first time. Do that and the money will follow.