I am not usually this single-minded but my hair needs colouring so it's on my mind. Another weapon against colour fading is this. John Frieda Brilliant Brunette Colour Glosser (or it's called something like that).
You shampoo and condition your hair as usual. Squeeze it dry (not with a towel or anything) and then apply this. Now it does make your hands a bit brown so work fast! And be careful as it can stain your skin. Honestly, what we go through for perfection...
Leave it on for 3-5 mins. Rinse. I know people have said this makes their hair greasy and non shiny and all sorts. My tip is not to put too much on - you really don't need loads, I put little bits on at a time as this means you don't splash it everywhere and you can control it better. And you don't need to use it every time you wash your hair (who has time to anyway?) I use it when I need to get my hair coloured but am trying to stretch it out a bit longer. Like right now. I will use it tomorrow. I also like the scent of it but it's quite strong, you can smell it on your hair at least 24 hours later.
Works for me!