Setting time of ZOE is best controlled by
The clinician can shorten setting time by adding a small amount of zinc acetate or additional accelerator or a drop of water in the paste before mixing or by extending the mixing time. Although these methods affect the property of the cement adversely, the use of a cool spatula and mixing slab is also effective in prolonging the setting time and is the recommended method to control setting time.
 
Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, 12th ed page no 179
Adding a drop of eugenol
Adding a drop of water
Cooling the glass slab
Altering ratio of two pastes