I do both, depending on what I missed and where I am in the week. I try to only plan one week ahead so taht I can take into account what else is happening in my life that week. On a busy work week, I try to keep my workouts to only what I can do and when work is lighter, I give a little more time to workouts, etc. More often than not, I just pick up where I left off and move on unless skipping it entirely will leave the plan "unbalanced." What I mean by that is if you missed one day of cardio and one of total body weights, then you can ditch those and move on. If you missed upper but not lower, then maybe you want to back it up a day or two to get both back.