If at all possible, keep chemically treated blonde hair maintained by a professional stylist. Talk to them about your preferred tone, and how you can manage the maintenance process.
Schedule your hair salon appointments at regular intervals (every six to eight weeks is average), but don’t overdo it by having it done too frequently. Also ensure that only the necessary areas are touched up to minimise damage to the ends.
Use a silver shampoo to keep brassy tones at bay, and a good nourishing conditioner to keep your hair in optimum condition. Also regularly trim dry ends to prevent grassiness and unnecessary breakage.