One of the achievements for Portal that I found to be a pain is Camera Shy because if you miss one, you have to go back and start all over again. What you need to do is go straight through all 19 chambers in Portal and detach all the cameras. Here's a list of how many cameras you should be detaching from each chamber.

Chamber 00 - 0
Chamber 01 - 0
Chamber 02 - 3
Chamber 03 - 3
Chamber 04 - 2
Chamber 05 - 3
Chamber 06 - 0
Chamber 07 - 0
Chamber 08 - 0
Chamber 09 - 0
Chamber 10 - 1
Chamber 11 - 1
Chamber 12 - 0
Chamber 13 - 3
Chamber 14 - 0
Chamber 15 - 5
Chamber 16 - 5
Chamber 17 - 2
Chamber 18 - 2
Chamber 19 - 3