No matter how hard you try, sometimes you just can't prevent getting exposed to poison ivy, poison oak, or poison sumac. Here are some strategies to help minimize your suffering after the fact.
Soap and water. This can prevent, or at least minimize, your reaction to urushiol, the irritant substance found in all 3 of these plants. Your best chance for success is to wash within 10-30 minutes of exposure. However, you'll still get some benefit from washing an hour or two after you've come in contact with the plant.
Some people believe that plain water is enough to prevent a reaction, so even if you don't have any soap it's worth at least rinsing as soon as possible. Just be sure to use lots of water to completely wash off the urushiol from your body and to avoid spreading it elsewhere. When I'm working in the woods or the garden, I make a habit of rinsing the exposed skin on my arms and legs with the garden hose every half hour or so.
Creams. Once the rash sets in, the main goal becomes finding some relief from the horrific itch. Several creams and ointments on the market can help in this department. Many contain some combination of zinc, menthol, camphor, rubbing alcohol, and other drying agents. Topical anesthetics like benzocaine or lidocaine can temporarily deaden the nerves generating the itching and so provide some relief.
Herbal and home remedies. Most herbal and home remedies are probably safe and may be worth trying if you haven't found anything else satisfactory. However, some of these can be harmful. For example, bleach can cause a severe burn if applied undiluted to the skin. I suggest avoiding anything that sounds risky to you and first testing home remedies on a small area of healthy skin before ladling anything on.
Hot water or ice. Very hot water seems to reduce the itching and burning sensation quite effectively, but typically only in the short run. You also must be careful not to burn yourself. Ice is terrific at numbing an area for several minutes or even hours. (By the way, you can't spread the rash by rubbing any of these topical remedies on the rash.)
Antihistamines. Over-the-counter antihistamines like diphenhydramine (Benadryl®), loratidine (Claritin®, Alavert®) and cetirizine (Zyrtec®) can sometimes help to reduce the itch. Diphenhydramine is also available in a cream that you can put directly on the rash.
Steroid creams. Steroids can be very effective in stopping the allergic reaction to urushiol. The simplest and safest form is the over-the-counter creams, most of which contain 1 percent hydrocortisone. This is a relatively weak steroid, but keep in mind that any steroid can cause many significant harms, so it's best to use the mildest form you can. If the OTC steroid isn't enough, your doctor can prescribe a higher-strength cream or ointment. However, these high-potency steroids should not be used around the face or genitals because they can cause severe damage to the thin skin in these areas.
Oral steroids. These potent prescription medications are usually reserved for only the most severe outbreaks of poison ivy, or when the rash has spread to the face or genitals. Some words of caution: If oral steroids are stopped too soon, the rash can come back with a vengeance, even worse than it originally was. You probably need a minimum course of 14 days of steroids to prevent this rebound effect, and most doctors prescribe a full 21 days for an extra margin of safety. Oral steroids can certainly stop the allergic reaction within the first couple of days of treatment, but subjecting your whole body to the prolonged, systemic exposure of these powerful medications carries much more risk of side effects than do the topical treatments.
Finally, a word about scratching. Although you can't spread the rash by scratching (once you've washed your fingernails well), raking over the rash with your nails can still make matters worse. If you scratch too hard or too long and tear the skin, an infection can occur that might require antibiotics, on top of everything else. The best advice I can offer is to try your hardest to avoid the urge, and keep applying topical treatments as often as necessary to make the itching more bearable. Usually, the worst is over in about 2-3 weeks.