For me the key is establishing mental trigger points that kickoff the response to doing something. Example: I only fill the reservoir with enough insulin to last 3-4 days so that the reservoir indicator on my MM 522 screen only shows 1 bar on day 3. That increases my awareness to doing a set change (just like the battery indicator running down) although I sometime extend a set usage to 4 days. I almost always change my reservoir and infusion set at the same time.
I tend to change my site and my insulin at the same time but I always write the next day I need to change my site on a mini post it and stick it on my glucometer. Then there is no way I can forget it. I also tend to think ahead and kind of prepare my week....if I know I have to change my site on friday, then I try to plan out the optimum time (ie. morning or afternoon). I always carry an extra site and an alcohol wipe in case I have to make an on the spot change. I did once and it was actually pretty easy...my car became my home for a few minutes!!! I will admit that I sometimes let it go an extra half day or so. I don't seem to have that sweet spot you are talking about - I just generally have much better control than when I was on injections. Hope this helps!!