It is often recommended to do a given number of sets of a given number of repetitions of weight lifts (regardless of your exact training methodology, which mainly dictates the number of sets and repetitions), and add weight when you are able to do that. I can see the logic in this, and it has been discussed at least once already on the site.
However, when exactly should you add weight? Let's say you are just barely able to finish the exercise with good form, whatever exactly "good form" means to you. Should you add weight the next time around, or should you keep working on that level until you aren't reaching total muscle exhaustion? Why? Does your personal level of fitness make a difference here?