If you run into a problem that you cannot diagnose, call for backup, search the internet, or discuss it with another computer technician. Even if you have the ability to remember almost every problem you have encountered, there are always new problems arising. The bottom line is, your customer wants their computer fixed, so use whatever means necessary to repair their problem.
An experienced technician or even one starting out should understand the importance of being prepared. Know the tools you will require and what you do not need to take with you on the job. Along with your basic tools including pens, a notebook, and paper, always carry memory with you as most people require more RAM.