Decrease the number of moving parts:
a) place your hand on the patient's cheek or nasal bridge so that unexpected movements on the part of the patient will not result in large movements of the removal device
b) instruct patient to keep forehead in continual contact with the slit lamp's cross bar
c) instruct patient to gaze at an object in the distance to further stabilize the eye
Source
Roberts: Clinical Procedures in Emergency Medicine, 4th ed