I was a redhead for five years prior to this weekend (went brunette). And yes, it is very hard to maintain. The key really is sulfate free shampoos. This is really important. There's a shampoo sold at Sally's called Colorgen and that is probably the most affordable sulfate-free shampoo you will find (Pureology and Bed Head also make them, but they are pricier). Another thing is to avoid washing your hair as much as possible. For me that was easy, because my hair is naturally very dry so I could go 3 days usually without looking greasy. But even if you must rinse it every day, try to shampoo only every other day at the very most.