Jeanise suggested using Milbon Replenishing Shampoo and Conditioner (sold at salons), to extend colour. The one time I washed my hair since the double process, I used a OGX Lavender Luminescent Platinum Shampoo. I haven't noticed much colour fade, so I'm pretty happy with the results of that drugstore option.
Since I heat-style my hair more often than I care to admit, I use Briogeo Don't Despair, Repair Deep Conditioning Mask (£30) as a weekly deep clean. I think my hair thanks me for it by staying pretty.