Hello Sacha.
I've been colouring my hair purple for a couple of months, using the same brand as you. I've been using L'Oreal Elvive Shampoo & Conditioner for coloured hair (in red bottles), have cut down on the amount of times I wash my hair and don't use water that's too hot. Seems to make the colour stay on for longer, but don't think there's much more you can do to make it last, although I'm by no means an expert on colouring hair, so will be interesting to hear from someone who is :-)