From GlobalSecurity.org (see here):
If burning particles of WP strike and stick to the clothing, take off the contaminated clothing quickly before the WP burns through to the skin. Remove quickly all clothing affected by phosphorus to prevent phosphorus burning through to skin. If this is impossible, plunge skin or clothing affected by phosphorus in cold water or moisten strongly to extinguish or prevent fire. Then immediately remove affected clothing and rinse affected skin areas with cold sodium bicarbonate solution or with cold water. Moisten skin and remove visible phosphorus (preferably under water) with squared object (knife-back etc.) or tweezers. Do not touch phosphorus with fingers! Throw removed phosphorus or clothing affected by phosphorus into water or allow to bum in suitable location. Cover phosphorus burns with moist dressing and keep moist to prevent renewed inflammation. It is neccessary to dress white phosphorus-injured patients with saline-soaked dressings to prevent reignition of the phosphorus by contact with the air.
Some nations recommend washing the skin with a 0.5-2.0% copper sulphate solution or a copper sulphate impregnated pad. Wounds may be rinsed with a 0.1%-0.2% copper sulphate solution, if available. Dark coloured deposits may be removed with forceps. Prevent prolonged contact of any copper sulphate preparations with the tissues by prompt, copious flushing with water or saline, as there is a definite danger of copper poisoning. It may be necessary to repeat the first aid measures to completely remove all phosphorus.