Good question. First, I tried to be realistic in setting my expectations in the first place (not always easy for me). Second, we do "regular" lessons 4 days a week and the 5th day is catch up so if someone is sick we can adjust accordingly or if something takes longer, no big deal. Also, the weeks aren't "dated" just numbered. So for instance, if we suddenly need a week off for whatever reason, no big deal, we just hold off and resume the next week. And my daily check-off sheets are on the computer so if ds is really stuck on a math topic and I need an extra week with it, I could go on to the computer, adjust the daily sheet and just move the math sheets to a new week.
It isn't a perfect system but I definitely feel more compelled to complete the work since I know what is waiting for us next week and changing things is sort of a pain so I don't unless I REALLY need to, not just because I got lazy one week (which sometimes happened before). ;)