You can’t fight the yarn and what it wants to be. Don’t try. A fluffy, woolen-spun, aran-weight wool is going to knit up differently from a tidy, worsted-spun, silk-linen blend. If you get into a project and find that the yarn you’re using is just not a good fit for the stitches you chose, swap out one or the other – either get new yarn, or pick different stitches.
One of the hazards when flying by the seat of your pants is that occasionally, you misjudge how much yarn you need. That happened to me with a lace edging. I thought I’d need two skeins, but it turned out I needed a third because I decided to increase the width of the edging. I couldn't find more from the same dye lot, so I spent the second half of the edging alternating yarn every other row to make sure the mismatched dye lots didn’t show.
It’s possible to reconstruct what you did after you finish knitting, but it’s so much easier to follow the notes you kept as you were working. It slows you down a bit, I know, but having done things both ways, I can promise you: it’s worth the extra effort. This is mostly irrelevant for the designers who write out their entire pattern before they ever start stitching, but we’re the sort of folks who wing it here, right?
If you, like me, like to design as you go, you might find yourself ripping back every now and then. That’s no big deal if you’re not on a deadline, but putting in lifelines can save you from having to go all the way back to the very beginning.
Designing an item completely from scratch every single time can be exhausting, and it opens you up to making mistakes in the pattern-writing process. I like to have a few templates for my most commonly-used designs, and then I can riff off of those templates.