Good post and thread idea! 
I definitely agree how taking photos before you dismantle or work on something is truely invaluable. I've learned this lesson all too many times the hard way. Only trial and error gets you out of that situation.
I suppose another thing I might add is the confirmation that you have all the right tools at hand to complete the project or else plan on buying it, whatever it might be. It is discouraging when you're working on something only to find out that you need some special tool that you don't have and have to call it quits until you get one.
Also, if you are working on some antique piece of machinery, do a little research and see what parts are available and what's not readily available, not too mention, their costs. It will make you an even more careful and cautious mechanic. I always like to think that I have a 'soft touch' when working on this type of thing, but I do find myself being extra extra careful not loose or break any parts that I know I will not be able to find again.
Depending on what it is, sometimes it's just best to go by the old rule of "if it's not broke, then it don't fix it!"
I'm always in the habit of trying to improve my machines one way the other, it could be anything really. While most of the time these little projects turn out to be a success, every once in a while I'll get into a pedicament and realize, "man, what have I gotten myself into!? Had I just left this alone or had a better plan of how I was going to do it in the first place, I wouldn't be having to spend this extra money to redo what I just did!" In other words, work within your own limits, have a game plan every step of the way and use good judgement. Always.
I definitely agree how taking photos before you dismantle or work on something is truely invaluable. I've learned this lesson all too many times the hard way. Only trial and error gets you out of that situation.
I suppose another thing I might add is the confirmation that you have all the right tools at hand to complete the project or else plan on buying it, whatever it might be. It is discouraging when you're working on something only to find out that you need some special tool that you don't have and have to call it quits until you get one.
Also, if you are working on some antique piece of machinery, do a little research and see what parts are available and what's not readily available, not too mention, their costs. It will make you an even more careful and cautious mechanic. I always like to think that I have a 'soft touch' when working on this type of thing, but I do find myself being extra extra careful not loose or break any parts that I know I will not be able to find again.
Depending on what it is, sometimes it's just best to go by the old rule of "if it's not broke, then it don't fix it!"