Gotta keep them clean, is the advice normally given. Google will provide many, many discussions on many sites on this topic, but the bottom line is that if you use shampoos designed for white dogs or use a Tide,w/ bleach substitute, etc, bluing helps a bit. The warning I use is that a drop of two of bluing goes a long way. DO NOT make blue birds, LOL. Easy enough to re-create that disaster.
Washed my White Rocks last year for the National Show and they came out fine. There are some realities with white feathered birds that you just have a) prevent completely in the first place or b) just do the best you can in cleaning them up and living with the results.