from personal experience dont bother with all those 'blonde' shampoos, they are over priced and made my hair look duller than ever before. i posted a similar question myself not so long ago as my blonde hair had got really dull and people suggested pantene ice shine and herbal essences. so i bought the herbal essences and somebody else suggested washing it twice instead of once (lather, rinse, repeat and all that), which worked a treat. so sometimes the simple options work best!
dont know if this helps:
http://www.theanswerbank.co.uk/Body-and-Soul/B eauty/Question325609.html