The toughest lesson by far that I learned from my adviser in grad school was to never stop thinking about the results of an experiment. He never said anything explicitly to this; it was all done by example. I've never seen somebody before or since that was able to get so much analysis out of a single experiment. Again, it was never stated explicitly, but his attitude was that if you've taken the time and effort to do the experiment, get all you can from it.
It was a great lesson to learn, as it is something that I perceive few other people learning it. Someone gives me a stack of data and some conclusions and I immediately go to work on what else can be gathered from it. It's not something that people are inclined to do - look for what else might be lying out there.
The pithiest slogan I've ever seen that captures this is: "A conclusion is the point at which your mind stops thinking."
So why was this the toughest lesson I learned in grad school? Because when I was finishing up and thinking I'm done, my advisor kept saying something to the effect of: "no, there is more analysis we can do with the results". It got to be quite unnerving. You keep thinking you can write it up and yet there is still more to do. Not more experiments like most people suffer from - more analysis, more thinking, more conclusions. When this is thrown at you at a moment of weakness ("I wanna be done and get out of here!"), you can't help but remember it.
So know you know why I was perplexed yesterday with what to do with that flask of polypropylene crystals floating in tetralin. We had already done the intended experiment and analysis, but there's that nagging voice that keeps pushing me to see what else we can do with it. I just can't toss it and I haven't yet. What to do, what to do...