It sure seems like there are as many diets targeted for psoriasis as there are for simple wieght loss. But one thing is almost guaranteed: none of the dietary advice for psoriasis is any more a 'cure' than any other treatment currently available. Going off these diets will result in a relapse to the disease state. Perhaps not until months or years afterward, but from what I've heard, anecdotally, it's just about a sure thing. Often, the relapse is quite quick, to the point where a single-meal 'cheat' can cause a psoriasis flare within hours.
So, just like dietary modification for diabetes, changing one's eating habits in order to treat psoriasis is a "treatment for life," along with all the other, more standard treatments. Diets won't make fundamental changes to how your body works, any more than steroids, UV, or coal tar will (just a few examples). The big differences, however, are that diet is extremely difficult to test (even in a clinical setting), that many of those who propose diets do so on theories that are, to be nice about it, not very sound, and that remissions gained from more accepted treatments tend to last longer without continuing treatment.
On the other hand, the chance that eliminating alcohol or caffiene from your diet will kill you is extremely low, but not all of these diets are so simple or easy. Some of them advise some fairly extraordinary things, including strict fasting. As always, before attempting any lifestyle changes which purport to be therapeutic for any disease, consult a qualified physician. This is no less important for dietary changes than it is for drug changes, as certain foods can interfere with drugs you may be taking, or can exacerbate other conditions you may have.
There is no solid evidence that any one-size-fits-all diet (OSFAD) plan (one which suggests that everyone with psoriasis eat these things, and avoid those things, etc.) does any better at eliminating psoriasis symptoms than any other OSFAD. Most of the proponents of the various OSFADs have performed no clinical studies, rely on testimonials, and appear to be concerned more with book sales than with their patients' overall health.
While there appear to be a few common topics amongst all the dietary suggestions for psoriasis to be found on the Web, many of them contradict each other. For example, some say citrus fruits can be eaten, while others condemn them. Still other will suggest that certain kinds of citrus are okay, while others are bad. The advice that can be found is all over the map, in this respect. Unless they're copies of each other, no two sites I've ever found have presented precisely the same list of dietary changes. As far as I'm concerned, that makes them all highly suspect.
The closest thing to a OSFAD that has evidence backing it up is the Gluten-Free Diet (GFD). GFDs are required by people who have such extreme immune reactions to gluten (a collective term for wheat proteins, including gliadin) that their immune system will actually attack their own small intestines, impairing the absorption of nutrients and leading to all sorts of nasty symptoms. This is Celiac Disease.
A handful of studies by G. Michaelsson and other researchers have shown that psoriatics who are abnormally sensitive to gliadin may get good results from a GFD. In one study, the researchers found that while 10% of regular folks have antibodies to gliadin above a certain level, 16% of psoriatics do. A follow-up study found that those psoriatics with that level of gliadin antibodies did very well on a GFD, while psoriatics who did not have lots of gliadin antibodies did not. These studies were pretty small, and largely unblinded.
If you've got a willing doctor, and the means to do so, there is (perhaps) a 1-in-6 chance a blood test will show that you've got increased levels of antibodies to gliadin. If such a test is positive, a GFD diet may very well help your psoriasis dramatically. But GFDs are very restrictive (wheat proteins "hide" in a lot of products other than the obvious foods), and so are probably not something you'd want to try "just to see." After all, if your psoriasis clears up coincidentally to your attempt at a GFD on the off chance it might do something, you might convince yourself, for no good reason, that a life without wheat is the only way you'll live with clear skin.
So I am most-assuredly not advocating that anyone simply run out and try a GFD to see if their psoriasis gets better. Because if, after months or years of regiliously following a restrictive GFD, my psoriasis came back, I'd imagine I'd feel highly betrayed and depressed at wasting my time.
If I were to advocate anything, diet-wise, it'd have to be a sort of "elimination diet," where suspected foods are eliminated, one by one, for weeks at a time, and the results - if any - dutifully recorded. The biggest problem with this approach, of course, is that it takes a long time, and any other changes in lifestyle or medications need to also be taken into account before anyone dooms anything to the "can't eat it" list. On the plus side, it's a procedure which creates an individualized dietary "plan" (in terms of what you can't eat, at least), and it doesn't make any money for any of the diet-book hucksters.