lasting hair dye

I have dyed my hair mahogany, red, purple, copper red, and blue-black.
All of which must have lasted around two months only. I use Garnier Nutrisse. (permanent)
Is this a fast-fading color product or do I do something wrong?
Someone told me if I took cold showers, and use less shampoos the dye will last longer. Is this right?
Does shampooing before dying your hair affect anything?
If I leave in the dye longer than 30mins. like it says, will it make my hair darker?
etc. etc. anyone can answer, that would be nice!

my friend used garnier too... she went from a medium brown to red... and it faded fast. all reds fade. i think you should try a different brand... try clairol or l'oreal they are better brands