All of the above answers are good and valid. I especially like the point about being humble and looking hard at what you might have missed.
- The only point I have to add is to look at Errata.
Often we read datasheets and specify parts for our design, quite early on in the project.
It can be a month at the very least before we actually have a fully populated board. :) During such time, often chip manufacturers find slight issues with their parts, and update the Errata for these parts. Most often than not, they usually specify a workaround for the problem which we might need to incorporate in our design.
Unfortunately, I learnt the hard way, after months of trying to debug an issue, read the datasheet over and over, and then found that I had not read the errata. :)