As mmckenna points out, get trained. We have used several training companies to renew our qualifications. The last training we received was from Technical Rescue Service, or TRS. It was a three day course. Most are two to four days and cover what you need to know. We are required to be tower rescue qualified, so we can self-rescue and/or rescue each other. Once someone falls and is hanging in their harness, it is a life-threatening emergency and they may not have the time to wait for emergency services to get on scene, set up, climb up to them and bring them down before blood pooling can occur. Our harnesses all have emergency straps that can be deployed that you can stand up on to relieve pressure.
My personal harness has those straps and a "preacher's seat" that allows you to sit and relieve pressure while working at height. It is a DBI Sala Exofit. I like it, but I still like the Miller harness from work better.