I weigh in whenever it occurs to me (often in the bathroom) or when I feel like it, but generally not more than once per day. There's not much point to me weighing more than once per day since my lowest weight is in the morning and I'm not going to punch a higher weight into MyFitnessPal later in the day, because it would just override the previous number (although on the rare occasion I weigh less later in the day, I'll override it—why not?)
I'm not fussy about what time I weigh, or whether I've eaten, and so on. I know my lower numbers are more accurate and my higher numbers and big jumps on the chart are just water weight from my salt intake.
I make sure I record a weight every day I weigh in, regardless of how high/low it is, and regardless of what else is going on. I weighed in every day while I had the flu (followed by bronchitis) a couple months ago and there's a weird little spiky part on my graph there, but I just remember it as "oh right, that's when I had the flu." All the data is useful to me, as long as it's taken in context. Next time I have the flu, I should have a better idea of what's normal for me.
One thing I don't do is tie any of my motivation to the scale. I do have a few goals for the future about my body fat percent (which is related to my weight), but for the most part my goals are based on logging calories consistently, which is something I have much more control over than my weight.
In contrast, my sister is calorie counting and weighs herself every week or so, but will only record her weight when she has lost weight. She says she feels bad about herself if she records any weight gain, but personally I think she's missing out on the extra enjoyment of losing every pound put on, and hitting a "lowest weight yet" despite the trials.
Each to his/her own, I guess.
