Hi Bev - blue or violet shampoo is a good idea... I would also consider investing in a shower filter as it could be the mineral deposits in the water causing the colour change. Here is a good article with some additional tips: http://www.drweil.com/drw/u/id/QAA400170
You might also consider either #4 or #5 here: http://beautyeditor.ca/2011/04/05/brassy-hair-you-has-it-but-here-are-5-neat-things-that-can-help-restore-your-formerly-blondelicious-locks/