This is my technique for dropping a currently airing anime:
If I skim through it, I'm not interested enough to watch it. It'll still be in my radar, because boring episodes DO occur, though.
This is my technique for dropping an older one:
If I don't finish it in one go. When it comes to older anime, I should have the means to get the entire series and not have to wait weekly. If something interests me, I will watch it through the end with minimal stops.
I do like your techniques as well, although what happens if you've not got the time to watch the weekly episode and this goes on until the next week's episode?