Take biotin and use Mane N' Tail shampoo/conditioner (not sure if its available in the UK) will make your hair grow better. Dyeing your hair with semi-permanent dye isnt bad for it, but the day before dyeing I recommend using a leave-in conditioner.
Also, to make your 'hawk look fuller, use a leave-in conditioner (garnier leave in conditioner is cheap and good) the day before you think your gonna put it up, and the day after you wash it out.
I know what you mean, my first mohawk got itself into a rectangle shape after about a year. It was around christmas time too, that the rectangle was at its most prominent, so all the family photos have a big squared off mohawk in them :(