I can't remember 1 single time when the crew was involved that we couldn't get a jumpseater on due to weight. The key to that equation is the crew being involved. If the gate agent has a notice that there is potential for a weight issue and the flight is full, sometimes they do not go out of their way to get the ops agent involved to investigate bin weights and the crew involved to help problem solve. The moral of this story, if you see a crew member let them know you are trying to catch a ride. Ask the gate agent or the ops agent to let the crew know you are trying to get on. If we don't know we can't help. Most of our agents will go the extra mile to help but not all. If you can convince someone to get the crew involved we can almost always accommodate a jumpseater.