Have you tried Dock Leaves ?
They're well known as a way of soothing Nettle stings and might help.
outdoors.stackexchange.com/users/53/Bernhard Hofmann
I use both cortisone and benedryl cream.
One reduces swelling, the other reduces the allergic reaction.
Make sure to only use water soluble creams!
Petroleum based lotions will block the skin from naturally expunging the poison ivy oil.
outdoors.stackexchange.com/users/54/Peter DeWeese
Benadryl cream works about as good as anything for me.
outdoors.stackexchange.com/users/61/xpda
I realize that an answer has already been chosen, but as a frequent sufferer of poison ivy, I feel obliged to answer.
The best way to treat poison ivy, is to not get it in the first place, with avoidance as the first step, but a blocker works as well.
The best blocker I've found is IvyBlock, which is a lotion.
However, I am more likely to use IvyX which comes as towelettes.
I keep a box in my truck and since they're handy and easy to apply, I'm more likely to use them.
However, once you get poison ivy, the best treatment I've used is a product called Zanfel .
It is a wash, in paste form, that bonds to the urushiol, then is washed away.
This stuff is amazing, but very expensive.
I have seen generic versions, but I have been reluctant to try them since Zanfel works so well.
Some folks have mentioned Benedryl cream/pills, which does help with the itching.
However, it does not treat the root cause of the poison ivy, the urushiol oil.
If I feel that I might have been in contact with poison ivy, I wash with dish soap (Dawn) as it breaks up the oil.
outdoors.stackexchange.com/users/12607/Tom
I believe ice can help.
It's a local anesthetic, reduces swelling, and closes the pores, keeping the poison out.
Of course, it can be hard to find ice..
outdoors.stackexchange.com/users/131/Shawn
Jewelweed should do the job, if you have it locally.
It usually grows close to the poison ivy.
outdoors.stackexchange.com/users/341/Sirex
Plantain weed not to be confused with the bannana like fruit.
Crush the leaves and rub it on.
It is usually growing on the edge of roadways or compacted trails.
outdoors.stackexchange.com/users/376/mikes
I'm not going to repeat any of the good advice given in earlier answers.
However, creams, lotions and salves will not work if you have a massive case of poison ivy.
Then there is nothing to do but go to your doctor or to a walk-in clinic and get a prednisone shot and/or tablets.
I had a horrible case of poison ivy many years ago --
hands, arms, legs, face, and, I don't know how, around my waist -- unendurable.
I got a shot of prednisone plus a course of prednisone tablets.
Within 24 hours, the eruptions and itching were totally gone .
Ever since then, I have been a fanatic about watching out for poison ivy and washing my hands thoroughly with Lava Hand Soap very soon after I even suspect I have touched it while gardening.
This has worked.
Prednisone is a powerful drug, and not one you want to ask for casually or use frequently, but when your body is covered with poison ivy, Over-the-Counter remedies are useless and you need prednisone or a similar steroid prescribed by a doctor.
outdoors.stackexchange.com/users/6411/ab2
Know how to identify it and avoid.
that is the only way.
Long pants and socks will help, but if you get it on them and use your hands in contact, surprise .. and then ... Homeopathic remedies sound fun but the only way to over come poison ivy is with the expensive soaps that enzymatically break down urushiol oil, the oil that causes the irritation.
Anything else is going to cause it to spread.
outdoors.stackexchange.com/users/6134/SkipBerne
If I'm struck by  poison ivy , what should I do? Is there a way to suppress the itch?
How should I treat poison ivy?
outdoors.stackexchange.com
<first-aid><plants><allergies><poison-ivy>
outdoors.stackexchange.com/questions/19
outdoors.stackexchange.com/users/46/Jasper
It is best to avoid poison ivy, but a blocker  such as Ivy Block does work. Cortisone and Benadryl cream are recommended. Prednisone is a powerful remedy in severe cases. Jewelweed, plantain weed and dock leaves can help, as can ice. Prevention is preferable, though blockers, creams, prednisone, certain weeds, leaves and ice can help.
Prevention is better than cure, but blockers, creams and prednisone are effective. Jewelweed, plantain weed, dock leaves and ice can be effective.