1)
It's very likely linked to something in the diet, first step could be to go completely organic and see what happens - unless the trigger is avoided, the psoriasis could return, and although there may be creams that would keep it down, I'd personally prefer to fix the problem rather than apply external short term fixes or take medicine.
2)
Traditional Chinese Medicine (TCM) will definitely help, but make sure you go see a proper chinese doctor (ie. there are TCM clinics on many highstreets, I use my local one for both myself and my 5yo. and 1yo. children, as well as hubby).
If you're in London, I can personally highly recommend www.instituteofchinesemedicine.org/welcome.htm, otherwise ask around to get a recommendation for a clinic near you.
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If you wanted to do a food trial, I would start by only letting your daughter consume truly organic food and drink, be strict with this, or your data would be useless and your time wasted, so read the labels on everything.
Note, that "supermarket organic" is not necessarily as organic as you'd hope - I can test this on my own skin, and my 1yo. daughter get asthmatic wheezing from "organic" bread from the supermarket :0).
I get organic foods at a local organic farmers market and local health shops.
I would particularly avoid all additives, even "natural colouring" etc. and in particular avoid artificial sweeteners (beware of the ones called something not indicating an artificial sweetener), and if possible, also all processed foods, which means cooking everything yourself for a month or two, to see if you get an improvement.
The point of this is to see if the psoriasis goes away, and then you can introduce foods again, to see which make the psoriasis flare up.
There are also some nutritionists that are aware of this, you could try finding one of those, or alternatively, search for "psoriasis" on eg. Yahoo Groups, http://groups.yahoo.com/ , to find other psoriasis sufferers and read the latest inside info.
The following site, for example, looks interesting: http://groups.msn.com/psoriasisisavoidable
All the best, hope you get some results soon.
Susanna