Replacing a shower's door strip is a simple repair that most older shower doors need. With time, the rubber seal that is the door strip becomes flimsy, failing to seal the door properly when closed. Any local hardware store or plumbing supply store will carry a replacement door strip.
Remove the existing shower door strip. Use a screwdriver to help pry out the strip and remove it as one long piece.
Clean away any debris in the track that held the strip.
Place one end of the new strip at the top of the shower door frame and firmly push the strip into the frame's track. You may need a screwdriver to press in parts of the strip.
Cut away and reaming lengths of the new door strip that did not fit into the track.