Clean the screen and position the measuring device: LCD panels don’t attract dust the way electrostatically-charged CRTs did, but it still settles there, and fingerprints won’t help your calibration efforts either. Make sure that if you have a colorimeter with suckers for attaching to glass CRTs that you don’t press it against an LCD panel as it could pull the front membrane off when you remove it.
All recent colorimeters for use with LCD displays ship with counterweight arrangements that allow the instrument to rest against the front panel. It usually helps if you tip the screen slightly backwards, to ensure that the colorimeter stays flush to the screen and doesn’t move during measurement.