Hi twentyten!
Dd loved every minute of it! There's another one here in London in July that she's going to as well 
How old is your dd? Is she dressing up?
Tips from dd are:
Take: camera, spare batteries, deodorant, drinks, lunch (vendors are expensive) and a secure bag for phone, money etc.
Ask before taking peoples photo, don't take a free hugs sign (!) haggle when buying - always works! Make sure items bought are genuine, don't be shy, chat to others - everyone is friendly, atmosphere is great, be prepared to queue, wear comfy shoes!
If cosplaying, try and make sure props are secure - if they break there is a repair station where props can be fixed, take a change of clothes just incase.
Anything else, give us a shout
Hope she has fun!