It has been said conditioner can weaken the knots. I don't use it.
For shampoo I use this :
http://www.salon-collective.co.uk/lanza.php?product_id=2267
It does help keep the colour & one of those bottles will last you over a year with a once a week wash. Its a UK supplier also.
The piece I am currently wearing was cut-in / dyed around 1st May. 4 months on & I have no colour issues which is most certainly down to the shampoo. Before using it I would get the reds creeping in after about 5 weeks of wear.