Besides avoiding it completely (learn to recognize the version of poison ivy, oak or sumac that grows where you live), the best prevention is to cover your skin. It doesn’t take much of the urushiol to cause itching and blisters; in fact, you only need to brush up against the plant to get a reaction.
By covering your skin, you lessen the risk of exposure. However, you must wash clothing after contact with the plant, or you run the risk of secondary exposure to the oil. It also doesn’t take much urushiol to cause irritation, and it can be spread from clothing to skin (even clothing to furniture to the skin).