I've been dealing with psoriasis since 2nd grade. At first it was a few minor spots, but starting in college I got really bad flare ups. No medication seemed to work. Then I started UVB light treatments. It worked, but taking an hour and a half to get to the place to stand in a machine for 2 minutes seemed so much work for something that I consider primarily cosmetic. I understand for a lot of people it's painful, stressful and more than cosmetic, but on my own personal cost/benefit analysis it seemed like overkill. So against the advice of everyone in my family except my DH, I stopped.
I did a lot of internet reading and a few things that I tried that seemed to make a difference.
Sunlight - The UVB treatments work for a reason. Obviously, too much sunlight is bad, but I try to sit outside in 30 minute intervals a couple of times a week. This is probably why your husband sees improvement when he in Florida.
B-12 & Folic Acid: I take 1000mcg of B-12 and 800 mcg of folic acid. I get mine in tablet form from Trader Joe's (it also has 2mg B-6). One little tablet that disolves under the tongue. I found out about this from the psoriasis usenet group.
Diet: Wheat sets my psoriasis off into a tizzy. It really sucks because Triscuits and Frosted Mini Wheats were my favorite snacks, but I noticed that a few days after eating them, the itching would drive me crazy. If I eat a lot of bread, same thing. Reading online, other people said that vegetables from the "night shade" family set them off.
Tumeric - Yes, the spice used in curry

The active ingredient curcumin, has natural antiseptic properties. I think mostly it helps keep infection at bay, which decreases redness and the itchiness. You can get it as a vitamin. Or start eating a lot of Indian food.