I see Southerngirl mentioned DOGtorX Formula. Yes it's worked wonders with my boy Zeus, who has to be the MOST allergy prone dog in the world. He had hot spots, sever break outs which even caused a pi derma fold on his foot but since being on DOGtorX Formula, not a sign of a break out.
Purina isn't good!!!!! I see a link has already been posted on this. Vets, unfortunately are not good dieticians, they just go with what the major food companies tell them, which is of course their particular brand is best.
I feed my guys mostly on raw meat (Kangaroo it's yucky for me but good for them) and also Supercoat as a dry kibble.
Zeus also has Cushings (don't panic I doubt Prissy has that) and DOGtorX Formula is keeping him well and happy. In fact he's now off the vet's medication, which only worked for a while then he went down hill FAST, and can now keep pace with Ra Kismet on walks, jump around like a youngster and is as energetic as a lazy dog can be
Here's a link (it's an Australian one but the product is available world wide)
http://www.petequinox.com.au/test.html