In life I am very structured so I carry that to my weight lifting. The day, what I'm going to do, the amount of sets, tentatively the amount of reps; all that I have decided before hand. And like Cord I use "instinctive" decisions while I work out.
I keep a log and I log myself in for my next workout to ensure that I work out. If I see I've been stagnant on a lift well such as the amount of reps or weight, I then write in what I want to do and I attempt that. When younger I must say it was easier to accomplish what I wrote down but now I find that much harder but doesn't stop me from trying.
In the older days I used to time myself between lifts and that was consistent however now I go by feel. On a lazy day I will take more time whereas on a day I have vim and vigor, I push the limits.
For days off, I agree sometimes it does give you time to reflect and appreciate weight lifting wanting you to come back at it harder. I usually take every stat day off; a fringe benefit I enjoy. I only work out during weekdays and in the past when I get my holidays at work, that is when I take a week(s) off lifting to re-energize.
I say do what works for you.
"IF I COME ... I'M BRINGING THE PAIN WITH ME"