First, place a towel or some other thin cloth (about the thickness of a pillow case) between the iron and the document - allow it to get as hot as possible with out burning the towel/cloth and do not allow the documents plastic laminate to start curling. If it starts to curl you could damage the document.
Step2
Once it gets hot enough you should be able to peel off the plastic, with out damage to the document. You may need to repeat the process several times.
Step3
If that doesnt work for you, or your lamination was done by simply melting two pieces of plastic together, in which case there is the "air line" you can simply cut the document along that lines and peel.
Tips & Warnings
Repeat step 1 as often as needed, often times targeting sinlge areas with the iron so that you can maximize the heat.