Next: running a debugger. I use print statements, but seriously, what's the difference? You use watch points in the debugger. Same thing. Once I have tests I find it easier to run them and look at the output of my prints as opposed to stepping through the code. Stepping through the code requires you to remember what you have done before and what the output was (granted debuggers that allow you to go backwards are helpful). If you can do that, then it's all good, but I find that it's easier for me to essentially create a log and read through it. It's just a bit more structured, but in the end it's exactly the same thing.
I think the biggest mistake that less experienced programmers do is that they don't try to reason about the code before they start. It's that isolation that's key, not how you are displaying the state of the program. Single stepping is fine when you've got 100 lines of code, but when you have thousands or millions of lines of code, it's not going to work. You need to be able to work your way backward from the error, reasoning using the source code as a map. You then use debugging tools (or printfs) to narrow down your options.
