I'm not saying I'm never guilty of any of these sins. Quite the opposite. In fact, I learned all of this from personal experience.
1) Buying OEM parts
Sometimes, you are dealing with a part that seems so strange and obscure, that you just assume that no aftermarket manufacturer would bother to make them, so you just go to the dealer and pay. Big mistake, especially in the age of the internet. Perhaps my biggest failure in this area was these Honda front stopper mounts. (motor mounts, kind of) I purchased these from Honda for $80 apiece, and later, came across them on the internet for, are you ready for it, $14 apiece. I don't know whether I was more surprised that some other manufacturer actually made them, or that they were so much cheaper then OEM. Take the few minutes to check for possible aftermarket alternatives before going and bending over at the dealer.
2) Buying junkyard parts without checking the price of new.
I know it's hard to believe, but often, you can buy brand new parts more cheaply than salvage ones. These Toyota truck turn signals are a good example. The thirty year old salvage ones are going to be faded, cracked, and brittle.
I once had Auto Glass Specialists come to my home, replace an broken side window on a Ford Tempo with new glass, and clean up all the broken glass inside the car, for less money than the junkyard wanted for a used piece of glass that I would have had to install myself.
3) Not doing research/not having reference materials
If you own the car, buy the book. It's that simple. If you work on your own car, and you violate this rule, you will pay for it - much more that the $15 that the book costs. These books aren't perfect, but they beat the shit out of winging it with nothing.
You Tube videos are also a great source of info, and actually show you how to do the job. Sometimes the books can be confusing. Watch a video, and you will be saying, "Oh, that's what the book means."
Videos are great, but not a replacement for the books. When you're down and dirty, in the middle of a job, it's a lot easier to refer to the book than trying to type on a keyboard with greasy fingers.
Every minute you spend, doing research on how to do a job correctly, will save you five to ten minutes (if not more), actually doing the job.
4) Not having the correct tools.
Don't be such a cheap fucker. Buy the tools you need. You're doing the job yourself to save money. What are you going to use that money for? Buy the tool. It will save you time, and prevent damage to parts that cost more than the tool, and in the future, you will have that tool. This is where the real payoff is. You will never have to go through the "Should I spend the money on the tool dilemma?" again, and you can tell you're friends that they suck because they don't have shit for tools.
5) Buying new instead of junkyard parts
Wait a second, I thought you said it was a mistake to buy junkyard parts instead of new.
It is, except for when it's not. Sometimes, salvage parts are dramatically cheaper than new, plus you can often get an entire assembly for less than the price of the one needed part. This can save you the time of tearing down and repairing an assembly.
Taking out parts at "pick a part" yards can also give you insight on what to do, and what not to do, when disassembling your own car. Better to break something on the junkyard's car than yours.