Entering a row full of people is always inconvenient, but you can minimize the hassle. Begin by saying "Excuse me" to the person on the aisle and then down the line, and "Thank you" to anyone who moves or stands to let you pass. Face the front or the stage. This means showing your backside to the people in your row, but should you trip or stumble, it's easier to regain your balance (and maintain your dignity) by falling forward against a seat rather than into someone's lap. A sort of semi-shuffling gait enables you to move forward without stepping on feet. Proceed as quickly as you can, and be sure to hold purses, cases, backpacks, and coats high above the people around you. Be very careful with umbrellas and canes.
If you must leave a row during a performance, repeat the process but whisper your thanks and apologies. If returning to your seat will inconvenience people yet another time, it's often best to sit or stand at the back of the hall until intermission and then get back to yoru place before others return to your row. Anyone who may be summoned away during a performance should try to get a seat on or near an aisle, as should parents with young children.
When you're the one being passed in a row, it's often possible to clear space by remaining seated and turning knees in the direction the person is moving. Standing may be easier if seats fold up, but it's better to move knees than to stand (thereby blocking the view of those seated behind) when the performance is under way.