Oh my.. the most important one is right... would you believe me if I said I do that here and assumed it was common knowledge ?
Which is a big problem with online help with these things. Omitted details which are 'common knowledge' may not always be common to everyone.
An important step that's often overlooked, is to verify your tester is working before you test the circuit your working on.
One could test a circuit your working on and say to oneself "yep, power is off" when your tester could be indicating power off on a live circuit.