Well, medicinally you can do benadryl and fish oil, they have a synergistic effect. My Snowball is 50 pounds and can have a max of two benadryl pills (get the generic kind they're cheaper and just as effective). I give her fish oil tabs 3 times a day and also drizzle about a tsp of flax seed oil on her food. It takes about 2 weeks for the results to be visible, but you'll notice that she stops itching almost immediately (gosh I love my vet .. she doesn't sell you unnecessary pills and other crap). If that doesn't work, you can ask your vet for prednisone (sp.), but there are some significant side effects which you should first discuss with your vet.
Raw honey is also one of those things that can help too ... there's no scientific evidence to prove it, but I've found it to be slighlty effective. Try and get it locally.
Also, make sure your pet is on a reliable flea preventative (Bio Spot On and Advantix work wonderfully) and to immediately get the fleas off you can use a CapStar ... and as stated above .. the food .. that's huge. Try a lamb and rice formulation from a good company (California Natural, Evo, Science Diet, Diamond is highly recommended by my sister in law, the vet, that's what she feeds her dogs ... Nutro...not Eukanuba, Ol' Roy, or any of those other companies). I personally have had good experiences with Nutro, California Natural, and Diamond. Evo is exceptional, but a lot of dogs can't handle it .. it's very very rich. Science Diet is nutritionally perfect, but unless a dog is raised on it, they probably won't eat it (I figure it's probably like us eating super healthy nutritionally perfect food). Corn is a common dog allergy, and if you're buying cheap food, it's probably one of the main ingredients. Def. follow the advice about gradual food change over .. otherwise you might have some stinky results .. plus if you have a big chested dog .. well we all know about bloat.